I understand that on a VPS, you're likely better off with a 32 bit OS as 64 bits have memory implications. Commonly, one will get a VPS with less memory to save money and hence you want a smaller memory foot print.
The problem is that this particular architecture (x86_64) uses more memory than plain old x86. Significantly more.
The other answers about your MacBook Pro and 64 bits are right. I have nothing to contribute to them.
The additional question I have is are you trying to replace Mac OS X or are you wanting to Run Ubuntu in a virtual environment? If this is the case, you needn't worry so much about 32 vs. 64 bits unless you are trying to do something specific.
If you need to pick one, I'd likely pick the 64 bit for a virtual environment but for such usages, I doubt it'd ultimately matter to me.