"Which is Better - 32 or 64 Bits?"
https://help.ubuntu.com/community/32bit_and_64bit#Which_is_Better_-_32_or_64_Bits.3F
(引用)
...
Unless you have specific reasons to choose 32-bit, we recommend 64-bit
to utilise the full capacity of your hardware.
...

上で参照されてるベンチ(9.10と古いけど)
"Ubuntu 32-bit, 32-bit PAE, 64-bit Kernel Benchmarks"
http://www.phoronix.com/scan.php?page=article&item=ubuntu_32_pae&num=1