You should make your decision based on the size of the servers you want.
32-bit AMIs only run on m1.small, whereas 64-bit AMIs only run on m1.large and m1.xlarge instances (plus the respective High CPU variants).
Larger instances obviously cost more money, with price theoretically scaling linearly with performance. However, I've found that large and xlarge instances actually have a little more performance/dollar than small instances.
Instead of being due to 32- or 64-bit architecture, this seems to be because of contention on I/O controllers. If you are doing serious work on EC2, I would suggest going with m1.large instances, and also the 64-bit AMIs that that entails.