Difference in performance, if any, in 32 and 64-bit versions of Ubuntu?

The only big advantage would be memory. There is a lot of hassle getting a fully running 64-bit GNU/Linux system working the way you want due to compatibility issues with applications and codecs. Phoronix did a test a little while back and there were no significant differences between 32 and 64-bit systems with similar resources. The compatibility hassle in itself would be enough to make me steer clear of the leap, however if you do have more memory, 64-bit Linux can allow each application to use more than the 3G limitation imposed by 32-bit Linux. If you use some of the resource intensive development tools, letting each use more than 3G of RAM may be beneficial. However if you simply want more memory and none of your applications require >3G, you can enable PAE in 32-bit Linux.

"To add to the above, 64-bit compiled programs actually use more memory:

The main disadvantage of 64-bit architectures is that relative to 32-bit architectures, the same data occupies more space in memory (due to swollen pointers and possibly other types and alignment padding). This increases the memory requirements of a given process and can have implications for efficient processor cache utilization.

So if you don't have above 4GB of ram, don't go 64-bit. You will end up using more of your RAM to run the same programs.

And of course, use the Ubuntu LiveCD for 64-bit/32-bit and try for yourself."