tough question ... X2a worked fine and stable for me for months; X3 is not quite there yet (but that may be me and my system; there are lots of people with opposite experience in X forum)
as regards vs-700 I haven't noticed much difference in performance (some
here but that was X3b)
anyway, you can run X2a and X3 in parallel (although I'm not 100% certain that's really the case as X3 overwrites some common files and dlls from the X2a installation)
BTW, don't install the vs-700 part of the x3 installer; i believe this is actually older than what is available in the support pages (more
here)