I tend to agree with uzi9mm, the XE Intel's use a Level 3 cache for the 2MB, which I have read is much slower (although still technically speaking very fast) than the L2 cache.
The newer 600 Intel's have the 2MB on their Level 2 cache, so they will generally run faster, and it makes their price tag cheaper. The 64 bit option is just the icing on the cake.