Hi I have a few Ares G36's and I highly recommend these. Others may disagree, but the build internally and externally are great. TM make great G36's but the shiny body puts me off, and the price.
The Ares G36's have a quick change spring guide, fold the stock round, remove the rear pin & cover and it's there.
Ares G36's also has the microswitch style trigger, they also do a EFCS (Electronic Fire Control System)
where it has a mosfet build into the gearbox & has a 2rd burst setting. These EFCS gearboxes are LiPo ready too.
Oh and the Ares G36's have electric blowback which is a nice gimmick, but does no favours for performance or reliability, still nice to have though. It can be enabled/ disabled easily by removing the recoil slide in top of the gearbox (1 screw).
Another great feature in the Ares ones is that you can change the outer barrel to fit a longer 'K' or 'E' version barrel as it has a barrel nut like the real thing
You'll find that the Umarex branded G36's are either OEM Ares or S&T. If it's the sportline one, it's most likely the inferior S&T brand. Umarex only license the G36's they don't necessarily make them.
Just my thoughts on the Ares one as I have alot of experience with it.
Ta