I've been looking at this myself, there is some info on the Yale website. basically the X-tech is the latest generation of high security locks from Yale.
The X5 is a 6-pin rim cylinder for a nightlatch, etc and the X6 is a 6-pin Euro cylinder and they retail for about £10 and £20 respectively (the latter depends on size, finish, etc).
From the Yale website:
X-Tech cylinders exceed the highest European security grade CEN 5 BS EN1303:1998. Antipick and antidrill features with hardened steel pins and ball bearings to protect the lock
Hope this helps