64 bit in Merom vs 32 in Core Duo.
4 MB Cache in Merom vs. 2MB in Duo
Higher clock speeds in Merom
More power per watt in Merom
Less heat in Merom
Overall, if you need a computer for more than 1 year or 1.5 years, wait. If you're like me, get something now. It all depends on how long you want to keep the computer and what you do with it.