I tend to install 32-bit Linux', because I've seen more people with odd problems with 64-bit systems than 32-bit systems. But a friend and colleague brought this to my attention:
Multi-core and multi-CPU support in 32-bit kernel
To guarantee compatibility with non-PAE processors, the 32-bit versions of Linux Mint Debian come with a 486 kernel by default. This kernel does not support SMP, and as a consequence is only able to detect one core and one CPU. If your CPU has multiple cores, or if you have more than one CPU, simply install the 686-PAE kernel and reboot your computer.
So: Better to install the 64-bit version, or install the 32-bit version and install the 686-PAE kernel?