shmups.system11.org

The project originally started on Twitter. I'm posting this here for posterity's sake. Maybe you guys have some more additions as well.
I wanna thank the following people who have contributed to the list (in no particular order): @PearlescentX, @Arasaka9, @JaimersSTG, @SPE2500, DBS, @SGP009, Ako, sikraiken, Gunner, Despatche

No cosmetic differences - only gameplay differences!

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Air Gallet
starting with different bombs (the respective other ones can be picked up later)

Darius Gaiden Extra
2p activates a mode where you play through every single stage

Deathsmiles
2p: weaker bomb (https://www.nicovideo.jp/watch/sm27478219)

Dodonpachi
1p: you can continue your chain during a laser bomb when your max bonus is active
2p: 1) small star items accumulate over the course of the whole game; 2) easier 2-3 midboss with laser ship types; 3) only fast bullets during final attack of the TLB

Dodonpachi Daioujou (white label and black label)
2p: less iframes during hyper activation on bosses

Espgaluda
2p: more disc enemies / bullets during st3 midboss [in the port, both sides behave as arcade 2p side]

Esprade
2p: 1) higher starting rank; 2) killing enemies with your barrier shot increases the bonus timer

Fire Barrel
2p: life bonus after each loop is doubled, even though the game only displays a single life bonus on screen

Forgotten Worlds
different characters
2p: more enemies [unconfirmed]

G.Darius
2p: bosses have more HP

Gekirindan
2p: 1) Type C has a stronger shot; 2) lower difficulty [unconfirmed]

Giga Wing
2p: shot deals more damage

G-Stream G2020
2p: faster movement

Guardian Force
different ship types

Hotdog Storm
2p: faster movement

Ibara
different ships

Ketsui
2p: slightly lower rank, barely noticeable

Ketsui IKD Special
2p: no extra lives after the completion of a level

Kyukyoku Tiger II / Twin Cobra II
green weapon is unique to each player side

Life Force
different order of powerups on the gauge

Mushihimesama
2p: 1) third attack of the TLB changes from aimed to random and is therefore more difficult; 2) second attack of the TLB is slightly different and has missing bullets (cf. https://youtu.be/UVAq597JFD0)

Nostradamus
different ships

P-47 Aces
2p: 1) due to a lower spawning point on screen, you can go inside the ship on stage 4 (cf. https://www.dailymotion.com/video/x4wa8yt); 2) you can bomb again after a bomb much quicker. This has no effect on the iframes.

Progear
bullet sound differences (https://youtu.be/dkRGZgwfnkw)

Raiden
2p: easier difficulty

Raiden II
2p: 1) starts with cluster bombs; 2) checkpoints (Japan ROM only)

Raiden III
2p: faster diagonal movement [unconfirmed]

Raiden DX
2p: After your hitbox increases from collecting a certain number of power ups, your horizontal movement becomes faster while your vertical movement becomes slower (https://streamable.com/cll1p).

Rapid Hero
2p: lower difficulty [unconfirmed]

Salamander 2
2p: lower rank

Sand Scorpion
2p: shot deals more damage

Side Arms
2p powers are different in combined mode

Sonic Wings
different ships

Space Bomber
one of the secret ships is different

Strania - The Stella Machina
with two swords equipped, the mech's third swing of the swords is different

Super Space Fortress Macross
2p: faster movement

Super Space Fortress Macross II
2p: faster movement

Thunder Dragon 2
different ships

Trizeal
2p: 1) faster movement; 2) gap in laser shot is narrower

US AAF Mustang
2p: 1) lower starting rank; 2) some enemies only give points on 2p side, e.g. the brown tanks in stage 8 (30k each) or the 4th boss

Varia Metal
second button is different. You either get a bomb (1p) or a shield (2p)

Zing Zing Zip
2p: slower vertical movement, faster horizontal movement

Very interesting - thank you for compiling!

Are the Cave ones generally considered bugs or are they likely intentional? The Mushi TLB stuff is weird, as are the extra enemies in Espgaluda and the DDP differences. Things like different ship capabilities makes sense, tweaks to seemingly random individual attacks are puzzling to me.

The first Sonic Wings has different Characters for 2P.

Well, the only difference that I know at the moment is...

Strania - The Stella Machina
1P/2P - With 2 swords equipped, the mech's third swing of the sword(s) is different per character.

Some (maybe all) weapons behave differently on the 2P side in Guardian Force.

In Space Bomber one of the secret ships (the one that's not the Tetris block) is different on the 2P side.

In Raiden II and DX (and also 1? I don't remember for sure) the 2P ship has faster horizontal movement but slower vertical movement than the 1P ship.

While I do not know the difference of either side, I do know that several differences when 2 players are active.

Strato Fighter - 2 players active
More power ups dropped
More enemies pop up

Chimera Beast - 2 players active
More creatures
Increased aggressive behavior(?)
Bosses have more hit points

Trizeal - player 2 ship is faster, gap in laser shot is narrower

Referring to the secret codes from the website, only one player can take the Tetris, yet both players can become the hidden ship.
https://gamefaqs.gamespot.com/arcade/568773-space-bomber/cheats

1P and 2P are essentially two different shot types in this. Sadly you can't select what shot type to play as such as playing the 2P version on P1 side.

All the weapon/bomb types behave differently depending on if you're playing the 1P blue tank or 2P red tank. For instance, the basic rapid weapon is tighter on p1 side and fires a concentrated laser as a bomb and is wider on p2 side with a 5-way beam as a bomb.

Pretty much the same thing as Thunder Dragon 2 - 1P and 2P are totally separate ship types.

Raiga: Strato Fighter - No gameplay differences between 1P red and 2P blue, but if you're playing 2 player there are about double the enemies and extra powerup carriers that appear. Not sure about any boss or second loop changes if any.

Do any of the Darius AC games aside from G. Darius have differences on the 2P side? It doesn't seem to be the case in the arcade, but Tiat is easy mode in a few of the home versions of Darius II.

The first Darius doesn't let you select 2P separately. II does, but I don't know of any changes. Same for Gaiden. Gaiden Extra Version, however, has two different modes for the player sides.

Do you have any details on this?

1P is the standard game choose your route gameplay (which zone is which level are shuffled around in the Extra ver). 2P activates a mode where you play through every single stage, as in this run here: https://m.youtube.com/watch?v=HP-Nr4FqTH4

Please add Side Arms.

2P powers are different in combined mode. (precursor to Forgotten Worlds)

Super Spacefortress Macross (the first arcade Macross I think, the one made by NMK) as well as G-Stream 2020 both have a 2P side that has slightly higher movement speed over 1P side.

The 2P boards of Same! Same! Same! (including the various Fire Sharks and the recently discovered Jiao! Jiao! Jiao!) have weird behavior for the player 2 side. At least the blue weapon does more damage, and only to regular stage enemies (not bosses). I can't really tell if the same thing is happening with the red or green weapons.

The player 1 side getting 0 points for the brown tank boss in USAAF Mustang stage 8 also applies to when that boss actually appears in stage 4.

Gekirindan's 2P Type-C shot type has doubled amount of 1P Type-C which makes the difficulty is lower for this 2P specific type. Other ships with 2P side is same as 1P.

I like to think that the "lore" reason is that there's two pilots for 2P's Type C in Gekirindan, so they have twice the power!

Courtesy of ZPS, regarding Same! 2P and Fire Shark:



The i-frames are the same, it's just that the 2P side ignores the bomb cooldown. For specifics, there's a 64 frame cooldown between bombing, and the bombs last 128 frames (the whole time you are invulnerable). If you press the bomb button on 2P side, it ignores the cooldown time and immediately restarts the bomb (setting the invulnerable time back to 128 frames).

Kyukyoku Tiger II (Saturn port only) - 1P side has a scoring glitch where it gets more points from powerups than it otherwise should

Alpha Mission 2/ASO II Last Guardian: P1 starts with the LASER armor in stock while P2 has HOME.

Blast Wind: different ships (though in practice, P2 is plainly weaker than P1)