I've got an Eagle chest rig and I just checked the double m4 pockets on it with my ak47 mags. One mag fits great if a touch loosely, you can fit two mags in there but it's a very tight fit, tight enough that I don't think I'd want to game with it. Perhaps if you left them in there for awhile the pouch might stretch some. *shrug*
As to vests vs. chest rigs, alot of people seem to prefer the vests, but in my opinion that more bulk then you really need. After all it's not like you need to carry more then mags and some water with you. We're not going on patrols for hours, miles away from base camp. And our lives don't depend on having a pile of different equipment on our person.
If you're uncertain about your loadout or are thinking you might want to change it in the future, definitely go molle. That will allow you to change the pouch layout whenever and however you want. You can get molle chest rigs but molle vests are much more common.
As to colors, If a TW variant or OD is your camo then OD or CB is probably what you'd want in LBE. RG would work as well. Tan not so much.