Have you had a go with the X5 Stu? I cant reccommend it highly enough, occasionally it even makes me look like I can drive (Batley national for eg)
I say stay with the B4 in 2wd, you know that car inside out, no wasting runs chasing the perfect set up.
Of course if anyone was to offer you HIGHLY DESERVED sponsorship (hint hint to anyone reading this) that may bias the descision.