Three phase will offer some harmonics cancelation of your nonlinear loads (I assume that there are mostly nonlinear loads, since you need UPS),, but you'll still have problems with “simetry” of your loads.. It actually really depends on type of your loads..
You're right. Most of the loads are nonlinear: mainly PCs and some other electronic devices. I don't think there is any problem with “simetry” of the loads if there is a correct planification of the distribution among the three phases, before starting the installation. The fact is that I'm trying to find out if there is any strong argument which makes one option really better than the other one, but so far there isn't.