I would go with 2 units. They are only controlled from a single PC, so it's not really ideal to have 2 PCs plugged into it. It won't be able to shut both down properly.
I have my server plugged into one of these:
PFC Sinewave Series - CP850PFCLCD and it seems to work well enough. It has kicked in a few times during thunderstorms and obviously during power outages. Doesn't make my PSU whine either, the previous cheap APC I had did, but it wasn't advertised as having pure sine wave output.