Most people would automatically say the slim is better. I've been googling around and see a lot of people saying both the Jasper revisions are actually the best 360s ever made. I can't find anything concrete.
I've never owned a Jasper. Are they any quieter than the older fats? All the fats I've owned have been really loud. I even put a Tailismoon "whisper" fan in one and it was still very loud, hehe.
Asking cause I found a really nice black elite Jasper v2 for cheap.