i think annie has hit the nail on the head, its all down to driving style... i have both cars and here's my opinion....
The B4 is a very good car and very easy to drive, it requires very little maintenance (i had mine for over a year and the diff is still as new, not touched it) i have been running it with a 6.5 brushless most of the season.
The B4 to me seems a very relaxed car to drive, and can take alot of abuse! i cant think of much else to write... Good car
The xxx cr is my preffered choice, again vey good car, requires a little more maintenance though (in my experience). i have only jus recently bought this car second hand towards the end of the season. but have run it quite a few times.
The car seems very settled but it feels more aggressive than the B4 to drive, it feels like as a driver, im doing more with the car and feels faster than the B4. again Good car!
as for your point made in you second question... the reason for people running variations on the chassis was (someone correct me if im wrong) that last year some people were running the BK2 which wasnt suited to the UK, id say that the xxxcr takes the best of both the original xxx and the BK2 and puts them together into making the xxxcr. hope that makes sense?
these are just my opinions of course, but i prefer the losi xxx cr to the B4
if i can help you any further on this subject, feel free to drop me a PM
Cheers,
Tom