It doesn't really matter imo. The biggest reason for why Vista takes up more memory is that it preloads some programs to the memory so they start up quicker. If you have less memory it preloads less programs. And as mentioned earlier. If you can't afford 2GB of memory these days then you can't afford a HTPC
So imo there should only be two things to consider:
1. Do you have a license for one of the OSs already? If so, use that.
2. What OS is cheapest to buy. (Vista can be had for as low as $50) these days.