What applications will you be mostly using? If the apps use all available cores (d render progs, video editing, etc) then go with quad-core. If you're mostly gaming, get the core 2 duo.
If the laptops are both the same price, get the quad-core for better resale value.
Personally I would want the dual core, better battery life and performance for games while it should also run cooler. But that is because I would want my laptop for mobile use because I already have 2 desktops with quad cores for doing cpu intensive tasks. This choice really comes down to what you plan to use the laptop for, it's hard to answer without knowing.