I haven't benchmarked them specifically but I don't think there's a difference in terms of I/O or CPU. At least I haven't noticed one (we've switched to a 64bit image a while back).

FWIW, we run a mid-sized deployment on EC2 (~60 instances) and anything interactive has to go on at least m1.large and up (usually xlarge). We do use m1.small's for queue-workers and low priority batch jobs.

Yes this is true on 64 but m1s. EBS IO is still very slow and m1s generally deploy to slower Intel 53xx and 54xx hardware (although this is still often faster than the AMD 2374 you'll get on Rackspace)