I have 2 G&G MP5s I'm doing work on right now...
I've done work on ICS MP5s before as well.
The G&Gs are better quality overall but they still have a few things that aren't good about em. The Motors for one are not very good, the fuses suck and they don't use a fuse box to hold them in. They have heat shrink tubing over the fuses. Which sucks if you need to change the fuse. Also the trigger switches tend to stick together.
All in all though the ICS guns are junk in my opinion and I'd go with the G&G despite the issues.
All the issues on the G&G are fairly easy to remedy.
Just my 2 cents.
Gumbercules!?! I love That Guy!!!