It's a tough call.

I'd recommend 64 bit since you have a multicore machine. If you don't do 64 bit, you may regret it later when you want 64 bit and have to install a lot of software again. You'll eventually upgrade your computer, so you might as well save that hassle now.

In the past, you'd have trouble finding some software for 64 bit OS on linux that you could readily find for 32 bit. Certain processes are faster on 64 bit while others are slower. Some things haven't been debugged as much on 64 bit. There's plenty of drawbacks. I think that linux kernels have improved enough to handle multicore better on either version so you may not need 64 bit.