To be honest, even with all the new options for 2wd's, I've not once thought about trading my x6 in.
In the right hands pretty much every one of the main 2wd contenders is capable of winning anything...B4, X6, Cougar, Tamiya. For me though, I find it easier to be consistently fast with the X6 than I ever did with the B4...I think that's because mid motored cars are more balanced generally than rear mounted motor cars, so it copes better with my inconsistencies
I did do equally fast laps with the B4 when I had a good day, but it punished me slightly more when I wasn't. 'Upgrading' to the Tamiya will feel much the same as driving a b4 I'd imagine. Same sort of driving style with subtle differences.
However, switching to mid motor car is a different driving style, so if it's a change your after with your 'upgrade' I'd suggest go X6 or cougar.
If you have a B4 already...then for me the choice would be obvious! You have some of the spares and all the tools already, so it's simples
[EDIT] Oh and S2's...for me the nicest looking 2wd I've seen...I'd love one of those...pity they're not made any more, I might have reconsidered the X6 for one!