Both are equally awesome. You just need to take into consideration the price and availability of parts to make your decision...
+1.....you will win with either one, but here are some facts I do know to help with your choice..
My XRAY feels indestructable! I cant tell you how many times, and how bad I have crashed this car, and have not broken anything! 6 months of practice and racing, no slop, no broken pieces. Cannot say the same for Mugen, guys at my track break em all the time.
The Mugen is more adjustable to set-ups than the XRAY.
Mugen has Pivot ball suspension-XRAY has C-hub, big difference to some, as the Pivot ball makes it more adjustable, but more of a pain in the butt to change A-arms, and you have added maintenance on the Mugen because the balls need to be kept clean.
The XRAY needs bearings cleaned or replaced every 2-3 gallons, Mugen does not have this problem.
Xray is slightly lighter, and a better jumper. Mugen puts the power down better, better traction and acceleration.
The Hudy spring steel on the XRAY make it superior quality, and the Kit goes together like a dream, no sanding, no filing. It practically built itself. You will not need any option parts, and the box set-up is as close to perfect as you will find.
I love my XRAY, but you have to choose what's right for you, and what your LHS stocks. Hope this helped, good luck