They are all good choices. All of them has their pro/cons.
Obviously, the Mugen is the oldest of the choices currently, and offers the worlds kit as a upgraded expense. Lots of Mugen buggies at the tracks.
Xray , although not as popular as the Mugen, does very well on the track. Offering both front ends in the current kit is a push on right direction for Xray. Expense is a setback, and parts available locally to you. Some of my fellow racers have the latest and the '18 version and love them. But you ask any faithful brand driver and they all respond the same.
The TLR 8XE is the newest of the trio, and has been stirring the pot at larger venues competing against larger odds and doing well. The 8XE has a very unique chassis setup compared to the prior 2 choices. The front end is quite typical for the C hub suspension cars. But the rear end on the 8XE is farthest from the norm on any 1/8 platform. The trans case is a upper and lower half unit, which still uses a offset input. While it does work , the user may see the tedious chore of accessing the rear diff for servicing on the bench either trackside or off the track. One awesome feature is the capability of swapping the shocks fore or aft of the tower. The kit provides all needed parts for the conversion. Along supplied in the kit are extra seats, diaphragm for the shocks and emulsion O-ring seals. The motor mount is brilliant in one way, and silly in other. The ability to shift the weight with the clamping mount works very well, but the adjustment for the proper ratios on larger tracks seems unobtainable with kit supplied parts. You'll get the typical 46t spur, and supplied 16t pinion which work. But fitting a 17t on the stock spur is troublesome. I like the battery tray, and chassis layout with the 8XE.
Now , there are optional parts out there for buggies which are users choice, and some can be quite expensive for little gain.
I personally own a TLR 8XE. I've been racing it about 2 months now, and I'm up to speed from my previous 1/8 platform, with very little changes from the kit supplied parts.
I have installed a steel 45t spur(not supplied) to get more rpms on larger tracks, which also in turn softens the initial torque naturally. I've installed the carbon lower arm inserts (not supplied) on the front only, filled the cavity with shoe goo for added flex reduction. I've installed bladders instead of emulsion(both supplied) and switched to 8 hole optional pistions(not supplied). Making slight adjustments every opportunity to hit the track, and it's getting faster and faster.
While it seems there are people breaking parts on the buggy, and the others also, I've yet to break anything. I do keep in my parts though, arm set, front castor blocks, rod ends, and velcro straps. Yes I've broken velcro straps during races .
I hope this helps, and if I can provide and info on the 8XE please ask, or venture to the 8XE forum here. Plenty of people talking about it. Good luck in your choice!