The B4 once setup right is a blast to drive, but, you need to have that setup right.
The XXX vehicles I have seen just seem to work with the setup more off than a B4 at times. Are they always the fastest, no, but if you spend more time fighting the settings, that can get really annoying and slow you down too.
I personally like the B4 over the XXX series, but won't argue with the fact that they are initially easier to drive fast than a B4.
RB5, haven't seen it at all, but would expect it to run well.
You could also look into an X6 or when it comes out the GV2 from Academy, looks to be a winner as well and if its anything like the SBV2 Pro 4wd buggy, will be highly durable.