you could always just buy the x6 conversion kit instead of a whole x6 if you already have a b4, it will probably be cheaper than buying a whole car like the rb5 or xxxcr, not going to start a 'this is is better...
' but can only say the x6 doesnt loose the rear end like a rear motor car like the b4 etc if you can give an X6 as much throttle into or round a corner as you like and it won't lose the rear end, it's planted and very forgiving if you make mistakes, I do think a B4 can be potentially faster over a lap if you get it right but the X6 is just easier to put consistant laps in throughout a race