Hopefully i can help in some comments here.
Had both the Xb4 v2 and Bmax4/3.
For the Xb4:
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1.building it was fun, had the chance to build 2 at the same time as was building the other one for my buddy. parts goes together perfectly, basically a very easy build. top it all of, Xray manual by far to me is the clearest and in colour plus point there.
2. Running it on our local track i did face some issue, the drive shaft pin broke through the collar not once, but 2-3 times for me, had to change the collar and it that point in time, Exotex collars are not out in the market yet. Did some thinking as since the drive shaft only had a pin inside and i dunno for what ever reason only the front keep breaking through the plastic collar, what i did was to use HOBAO Hyper TT CVD pins and grub screw to secure it..and there after...no more pins popping out from the the collars.
3. As for gear diff, XB4 uses plastic gears and they are extremely lightweight and perform well, but, i would invest in Xray Nt1(If i am not wrong) metal bevel gears just to be on the safer side as i believe metal does hold it own longer then plastic gears and to top of, its a 4WD buggy, the stressload on the gears are very high.
4. XB4 are very much like his bros/sis T2/3/4 which ever model xray touring cars, there are 101 ways to tune her to suit your driving and i have seen, currently one of our local drivers set her up to perform flawlessly, perfect flight, pefect landing.
For the Bmax4/3
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1. Jap standard, hassle free, straight forward, idoit proof, no stress build.
one thing for sure, the alu parts on japs cars always leave me in awe. beautifully crafted, well machined alu parts, no dings no dents. what surprise me is Yokomo does give you a piece of counter weight for you to place either Fore/Aft of the chassis, very nice touch from the yokomo factory.
Even the holders for the saddles can be shifted about 4mm Fore/Aft. Nice touches.
2. Virgin run, followed exactly the box stock setup, she handles like a dream, easy on the trottle, flys and land well, turn in mid corner able to punch her out without fishtail or power slide. had some issues with her shocks as she was leaking abit(yokomo drivers will know) so what i did was to change her O Rings to RDRP white O rings and holding good so far. added 1.6 and 1.7 pistons and she drives like a dream now. Had issues with the front lower hinge pins as mine was one of the 1st few batches that rolled out from yokomo, the hinge pins are very brittle and i broke mine on the 2nd outing. was told to use Bmax 4/2 lower hinge pins as they are more forgiving, which i did, no issue till now(yokomo had made the effort to swop the lower hinge pins for 1st batch owners)
3. Diff gears are Metal all round and yes, very simple to work on her if you need to maintain your diff or changing your diff oils.
4. Setting up basically just start from box stock, the Springs that comes with it are almost perfect for most track, i am still using the current springs as i dun see the need to swop for stiffer a/o softer springs, then again, to each its own on this matter. Again, tons of setup to personalize your driving, check out Naoto last EOS on his Bmax4/3 and you will know what i mean.
So, as above, you can see that for me as a novice club racer, there are many pro and cons on both the buggies. We have parts support for both XB4 and Bmax4/3 so on this part support matters, both are equally good over here.
it all boils down to my opinion is which brand you like thats about it, yes the xb4 overall weight is lighter, the Bmax 4/3 more stable and it goes on and on. and after this comparison is done, maybe you will head over to other manufacture tents to buy Team C, Tamiya, HPi, AE, TLR as maybe cheaper or more bling? and why did this comparison ever started?
so i guess its never an apple to apple, orange to orange against each brand. Each brands has its pros and cons and again IMO, it all boils down which branding suit you best.
cheers