I fly both the MVAM and the Defiant-R. Both are really, really good ships and you can't go wrong with either. I guess I'd have to slightly prefer the MVAM.
PVE: MVAM wins. That Lt. Comm. Sci slot makes for a very versatile ship. In STF's roll with gravity wells and save your PUG's butt when they ignore nanites/probes.
PVP: MVAM is a bit more offensive, Defiant-R is a bit more survivable. For the MVAM, use viral matrix (LTC Sci) on that fleeing BOP and watch it sit there while you alpha-strike it. For the Defiant-R throw on some deuterium burn when you get in trouble, gain distance, and cloak. Then wait for the right opportunity to get back in the fight.