Clarity and light gathering ability for sure. Too much magnification can be detrimental as it magnifies your movement, too, which can make it harder to focus the eyes. Besides, if you are shooting IBO sanctioned shoots then you are limited to 8.5 magnification. I do not know if the ASA has limits. You'd have to check their website.

I think a lot of dedicated 3D shooters are using 8.5x42. I'll tell you this, though. You won't find any of those $19.95 el-cheapo compacts on the IBO or ASA circuit. For compacts the best I've every taken a looky through is Leica, 8x22 I think. They're better than most 8x42 Japanese glass.