I've only played Daifukkatsu one time, and found it VERY easy compared with Daioujou, reached third level without any problems, but had to kill myself because my girlfriend was waiting. My first play with Daioujou was "Um, it's not as hard a people say" first boss "ok, looks harder than Dodonpachi" second stage "......this is getting harder" second boss "time to get serious" third stage "ok, I'll never beat this game"

. About gameplay, yes they play similar, but in DOJ, you have the hyper mode. The hyper mode consist, basically in a boost to the power of your ship, at the right side of the chain counter you have a bar that fills with the chain, when you reach the top, an item appears, and you can activate it with the bomb button. You can get up to five, and when you release the hyper, all the ones that you keep are activated, a side effect is that, every time you use the hyper, the game gets harder. Can't tell much about Daifukkatsu, I think that when you use the hyper you can destroy bullets, but I'm not too sure.
Edit: I'm not an expert, but hope it helps
