There ya go. Straight from the horses mouth.
I should add, some guns are better than others. Brand names aside, an AUG and an AK traditionally perform better than an M4 with minimal upgrading.
So like where you'd need to spend $120 on an M4, you could spend $60 on an AK and have it shoot just as well.
The differences in the different types of guns don't really show up until you start upgrading them. For the most part, most stock guns shoot on par with one another, and two fully upgraded guns (like an AUG and M4) will usually perform very similarly anyway.
As a gunsmith, what I look for in an AEG is simplicity. Sturdy metal body, 100% tm spec mechbox for upgrading. And none of that fancy crap like EBB or PBB that just makes the mechbox fail faster.