There's been alot of controversy about Core 2's 64-bit performance, and while I would go so far as to say that Core 2's 64-bit performance is fairly good and much better than the P4's, it's still not up to par with AMD's.

Also, it could be that AMD's 64-bit compilers are just much better than Intel's, or that the programs themselves were optimized for AMD64 alone seeing as AMD64 was the first x86-64 implementation, and not Em64T..

64-bit benches



All in all, even if Core 2's 64-bit performance isn't as fast as AMD's, 64-bit won't be much of a factor in the mainstream for 3-4yrs..