Just want to add my thoughts. I'm not very familiar with either exact amp, but I do own an HD150 Spider IV head which is the 150 212 in a head version. I play it though a 2x12 Marshall open back cabinet (G12T-75s). I've only played on a Spider Valve MII 212 at a music store.
For decades I've been a tube purist. I've always played through all tube amps because to my ear they've always had a superior sound to solid state amps. Recently however I bought the HD150 head as a backup to my JCM2000 Marshall head and when I tried playing the HD150 head at a rehearsal, the sound was so good that I decided to start using the HD150 as my gigging amp.
For me it's hard to completely evaluate an amp until I use it with a band. The HD150 has such an incredible sound to my ear (Diezal Herbert - higain green) when I play with drums and bass which goes against my tube purism. This is the first time I've ever preferred a solid state amp to a tube amp and the first time I've preferred anything to a Marshall equivalent. I'm sure that the Diezal Herbert that the higain green is modeling would be as good or better if I had one and it was cranked, but the HD150 Spider IV gets that power tube breakup sound at non-deafening sound levels that I need in most clubs we play.
I wish I would have played the Spider Valve mkii 212 more and with a band so I could speak more to it. It does sound great at all volume levels from what I could tell at the music store. I'm guessing that it might edge out the Spider IV 150 212 at higher volumes because of the real power tube breakup but at lower volumes it might not stand apart as much because both have Line 6's new excellent HD amp modeling.