He was asking for personal experience on them and be careful with ArniesAirsoft "overall rating" cause at times the reviews are written by anyone and someone can put a rating of 1 and screw it all up. I believe rexdlu had a G&G UMG
I personally like the MP5 (TM MP5, sorry I never used a G&G but its similar build) just felt right in my hands, but if you don't care about looks I would choose the G&G UMG cause it has a
Version 3 Gearbox
Which is slightly easier to work with than the MP5 Version 2 gearbox as well as the UMG comes with that Metal piece that holds the motor in place.
Whereas the MP5 you have to slip in the motor wires through two small holes and then connect it to the battery and there is a little screw that adjust the motor height which can be lost or you have to re-adjust if it gets loose.
If you are expecting to do your own modifications I would go with the UMG just because of its Version 3 Gearbox and that metal piece that holds the motor in place
UMG Motor Holder (Bottom of Pic)
Pic Courtesy Evike