This has nothing to do with virtualisation but if you can you should really try to avoid the 100 series. They really are cheaper in every possible sense of the word, and it shows.
We have had nothing but trouble with the three we have in use: one DOA, one defective system board and two cases of defective or incompatible RAM). Every incident cost me about three days on the phone, writing e-mails or debugging the hardware. HP support is great but it takes quite a while and many hoops to jump through before they accept that it's a hardware problem and not anything else.
On the other hand we have about 15 DL380 G6 in various projects (all of them doing virtualisation with KVM) and the worst that happened was a surprisingly large number (six or seven) of hard disk drives failing after a few months and one system board with a defective DIMM slot. (And no, the disk failures had nothing to do with controller firmware bugs.)
If you can, please go with the 300 series, especially for virtualisation. It will save you headaches and many support hotline calls. You also get three years of warranty for servers and hard disk drives with the 300 series as well as next business day support on site, without any additional support packs. IMHO those are killer arguments against the 100 series.