might want to look at KWA M4 also. Very reliable out of the box with no real need for upgrades except maybe a TB barrel. Has way more positive history and feedback then SRC although SRC is definately one of my choices. You'll find that the KWA gearbox is truely reinforced where it needs to be and the gears are even thicker then systema gears. From the research I've done they seem to be the best externally and internally for the price. Another thing I noticed is that they re-engineer their various models based on input from the players. Here's a link showing how many player suggested improvements they made between 2008 and 2009
Just a side note... the 2GX gear box is one of the few that doesn't need shimming right out of the box. Also don't believe all the bad mouthing you might hear about the 2 peice plastic hop up. The bad mouthing is from those that don't own a KWA and don't know how good it is. The materials and design was chosen for is tight air seal and accuracy it gave.