I've used both but I used them alone, not with MHs. T5s have a much better "punch" in that they can pentrate the water deeper then VHOs. If you're going to use actinic bulbs the PAR (useable specturm) is more or less worthless on them so the bulbs will be for you, not your corals.
IMO VHOs have a truer actinic then T5s but again I've never used them with MH bulbs. perhaps someone with more experience can chime in with their take. I know a few MR'ers have switched to T5 actinics from VHO to suppliment their MHs.