The biggest differences is that it is hard to get some drivers and some things
to work. I had Ubuntu 8.04 64-bit installed and got everything to work without
too much problem. However, some things were problematic. There is no Realaudio
client for 64-bit, for example. Lots of programs are not in the 64-bit
repositories and if you are doing anything outside of Ubuntu you need to be
careful. For example, if you are installing from a website such as Getdeb.net,
autopackage, Automatix or klik many things are either unavailable or won't work.
C'n'r from Linspire also does not work with 64-bit.

It depends on how you use your machine and what you use it for. I use lots of
multimedia and VMs, so did not see any real advantage to using the 64-bit and
given the unevenness of the support, I elected to stay with 32-bit.