I have rebug and Kmeaw installed on two different systems... pretty much solely use the kmeaw PS3 (slim). Did have a prob with the phat rebug on one occasion (rsod), which was pretty much my own fault. I'd go for rebug, just because its still being supported.
i'm new on the ps3 world, and i need some "lights", don't we have anything yet to break the fw 4.21? i saw something about firm 4.21 was nuked on youtube last week about it, but i didn't see in any forum news about it.