I only have first hand experience with XA2, so I can’t really make any recommendations on which device to get…
However, if you care about Android app support, don’t get the Xperia X (it still has the old compat layer, which only emulated Android 4.4) and the X10ii is the first 64bit device out of the bunch, so there may still be some teething problems that may be resolved in future updates…
In any case, I would recommend you have a look at the “Known Issues” section of the release notes: