I rate the X yacht very highly.
I've raced quite a bit on 40.7s and x's and personally feel that the quality of the X is far superior. No idea about detailed construction differences, but one thing that is pretty clear is that they can both take pretty heavy weather - with a full crew - and both have potential to be a handfull without.
X's apparently have a bit of a problem with losing their rigs, so I'd recommend that you do a bit more research on this. (we lost ours in 15 kts, but then it was probably something to do with the hard racing life it had had prior to that event) We did sail it offshore twice a month (through summer/autumn) for 4 years prior to the event, and in winds up to 45kts.