Welcome to the forum!!
From what i've seen, the CAS G&G's have a lower build quality than the "normal" versions. I would recommend a normal version (they don't have a smoked plastic receiver) as they feature pre-upgraded internals, (a tighter barrel, for instance) and are of sturdier, higher-quality construction. I'm not to sure about ICS, i haven't looked much into them, but Most of what i've heard has been good, and i think you'd be happy with any one of these guns. As for who to buy from, that would depend on where you live. Filling out your profile will fix that. Mach 1 airsoft is located in toronto, and 007 is in Calgary. So it comes down to who is closer, as whoever's closer will have cheaper shipping fees, and i beleive both stores carry pretty much the same guns, with only slight differences, aside from the CAS models. hope this helps!!