id get the 4wd if the class is strong enough at your track. i drove the xxx4 when it first came out and i loved it but its not a big class in socal.
the advantage with the losi is you can get the xxx-ke that have everything you need, but the b4 is not the factory team yet, you can buy all the stuff but spend a lot of money doing it. both buggys can win on any given day, and the two magazines that have had "shoot outs" between the two have both came to a draw on the results of which one is better.
can you drive someones b4/xxx to see which one fits your driving style? because that and your personal pref. will be the biggest factor.