I started out with B4, and swapped to an X6 2 years+ ago now I guess, I haven't regretted it at all.
As a relative newbie at that point, the simple comparison I found was that to drive the B4 I found it to be a bit 'point and shoot' whereas with the X6 I was rewarded for driving corners and lines, so a bit more like a 4wd.
By point and shoot I mean, it pivots quickly around the rear wheels, so found I was aiming for the corner, full throttle, full brake, then turn...and repeat! but never mix as it would swap ends too quickly and easily.
With the X6 (or mid motor), it pivots much slower due to the weight distribution, so I tend to drive smoother lines, with more progressive throttle/brake in and out of the bends.
Looking at my fastest laps with each, I was just as quick with the B4 on good laps, but really struggled to be consistent with it due to the pivoting! With the X6 my lap times are far more consistent, as the balance makes it more forgiving, and better suited to my reaction speeds!!
For me at least, that consistency has meant more confidence, which I'm sure has meant me going faster now overall with a bit more experience under my belt!!!
IMO none of the mid motor cars have anything significantly different about them, so none will be outright faster than another. In the right hands any could win big events.
Try as many as you can, and see what you like, you might like it! Still love my X6
