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!----------------------------------------------------------------------------------------------------------------
Air Galletstarting with different bombs (the respective other ones can be picked up later)
Darius Gaiden Extra2p activates a mode where you play through every single stage
Deathsmiles2p: weaker bomb (
https://www.nicovideo.jp/watch/sm27478219)
Dodonpachi1p: 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
Espgaluda2p: more disc enemies / bullets during st3 midboss [in the port, both sides behave as arcade 2p side]
Esprade2p: 1) higher starting rank; 2) killing enemies with your barrier shot increases the bonus timer
Fire Barrel2p: life bonus after each loop is doubled, even though the game only displays a single life bonus on screen
Forgotten Worldsdifferent characters
2p: more enemies [unconfirmed]
G.Darius2p: bosses have more HP
Gekirindan2p: 1) Type C has a stronger shot; 2) lower difficulty [unconfirmed]
Giga Wing2p: shot deals more damage
G-Stream G20202p: faster movement
Guardian Forcedifferent ship types
Hotdog Storm2p: faster movement
Ibaradifferent ships
Ketsui2p: slightly lower rank, barely noticeable
Ketsui IKD Special2p: no extra lives after the completion of a level
Kyukyoku Tiger II / Twin Cobra IIgreen weapon is unique to each player side
Life Forcedifferent order of powerups on the gauge
Mushihimesama2p: 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)
Nostradamusdifferent ships
P-47 Aces2p: 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.
Progearbullet sound differences (
https://youtu.be/dkRGZgwfnkw)
Raiden2p: easier difficulty
Raiden II2p: 1) starts with cluster bombs; 2) checkpoints (Japan ROM only)
Raiden III2p: faster diagonal movement [unconfirmed]
Raiden DX2p: 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 Hero2p: lower difficulty [unconfirmed]
Salamander 22p: lower rank
Sand Scorpion2p: shot deals more damage
Side Arms2p powers are different in combined mode
Sonic Wingsdifferent ships
Space Bomberone of the secret ships is different
Strania - The Stella Machinawith two swords equipped, the mech's third swing of the swords is different
Super Space Fortress Macross2p: faster movement
Super Space Fortress Macross II2p: faster movement
Thunder Dragon 2different ships
Trizeal2p: 1) faster movement; 2) gap in laser shot is narrower
US AAF Mustang2p: 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 Metalsecond button is different. You either get a bomb (1p) or a shield (2p)
Zing Zing Zip2p: slower vertical movement, faster horizontal movement