Here's the difference. With Ikaruga, it is quite hard, but you know you'll beat it eventually because the game tacks on another credit for every hour you play. The difficulty is done in a way where you're really pushing yourself to finish the game, but the game gives you a chance to actually get through it. I feel myself continuously getting better with every play session.
With F-Zero GX, there are just certain monstrous things about it that you know, your skills as a gamer, you will never, ever beat. I beat the Story Mode on Normal (aka Easy) but I can't even get past the FIRST LEVEL on Hard. I fear what Very Hard is like. Bear in mind I love F-Zero, that game is awesome, but I also know the way the computer challenges are set up I'll never have the twitch skills to make it quite that far, and it was hard enough getting through single player the first time. Arguably some people could probably never beat even the second level on Normal (the one with the boulders).
Ikaruga, things are nuts, but I think the timing and twitch skills in it are more forgivable over time. The game does ease you into it, the first level being far easier than Level 5. The question is, do you want to be the game on Hard with one credit? Also, the difference between Easy/Normal/Hard is quite drastic, almost feeling like a different game. See, that's a different stratification.