For me, the difference can be summed up in one sentence: KICKASS, despite its dark moments, is at heart a funny, feel-good movie; SUPER, despite its comic moments, is at heart a dark tragedy.
Anyone who thinks that one of these movies is a ripoff of the other hasn't seen them. They start from the same basic premise, then shoot off in totally different directions.
If you've seen one of them and liked it, I'm pretty sure you'll also like the other - but you'll find you have a definite favourite.
I like them both. Giving SUPER four stars instead of five is purely a matter of personal preference - I preferred the lightness of touch in KICKASS, and I really, really, REALLY wish that SUPER hadn't... um... no, I can't say more without a major spoiler.
I like SUPER, and I'm glad I bought it, and I'll watch it several more times, I'm sure. But each time, I'll probably watch KA afterwards to cheer myself up again.
But anyway, DEFINITELY recommended!!!