I cant choose ><

Nebula : love the "show only favorites" option, (got tons of roms so cant life without it ) but has kawaks also something like this option ?

Kawaks, I want to stay with this baby, it runs better, smoother, sound is better and great preview rom screen window. Has nebula also this preview screen thingie? I ask this because in nebula it doesnt work with me...

No, Nebula doesn't have a preview screen. It can use skins, but that's about it.

I can't really pick favorites. I don't like a few features of Kawks (like the difficult way of binding custom button sequences and the way you have to pause emulation before you can quit). Plus, Kawaks doesn't support the Universe BIOS properly.

Nebula supports the Universe BIOS and makes the aforementioned buttons bindings easier to configure.

I absolutely hate closed-source emulators, so I'd rather go with something like Final Burn Alpha (FBA). But not given that choice...
Nebula is slow and clunky, Kawaks is fast, but has some of the most annoying "features" I've ever seen.
I'd pick Nebula, it supports more games, high quality graphics filters, and I don't get pissed off just looking at it.