RE: Trex 500 or Gaui 550
I have both and they fly different. The Gaui is a nice heli, it does not have the power of the Trex 500 and has some issues that need work, then again so does the trex 500.
If you fly mostly scale and sport and want something bigger then the gaui is an excellent choice. If you want to fly 3D at some point in the future then the Trex is the better choice.
Just to add a bit on the design. For the servo design, a push/pull on a 550 and higher is the way to go. The direct to swash is fine and gives slightly quicker response but does not have the strength of a push/pull system. There is a reason why all the larger helis use push/pull and it's NOT about space. A bit harder to set up initially but once in place puts less strain on the servos.
The gearing is a little strange on the 550 but I've never had to replace a gear yet so don't know how much of a pain it is or not.
If you're budget conscience and want a larger sport flyer get the gaui, if you want a fast response 3D get the Trex. The Swift 16 is also and option in this size. Also in the 500 size look at the Hirobo Lepton EX.
Keep in mind that while Trex quality is good, they are NOT considered the best manufacturer, I'd definitely put Hirobo and JR above then and possibly Kyosho as well. Align does seem to have the best marketing department though...