If you only intend to support 10.5+ then 64 bit should be fine. Quite likely there will be no advantage to 64 bit over 32 bit AFAIK, the only way is to test performance in both. Some frameworks are not available in 64 bit (and some not in 32 bit) so think ahead to what functionality you might want to add in the future. Personally, I would be happy to be 64 bit only if I don't mind the limitation of 10.5+ only; Lion will be here soon.