Here's my take, fvs's are better heroic questers, they can handle any party healing and deal damage to the mobs better.
Radiant Servants are better healers. Back pre-motu I had a fvs that I tr'ed to a cleric as I primarily pulled him out to heal raids. For raid healing I loved having a cleric more than a fvs, the glows for light damage, bursts for a free mass, and still having mass heal let me be more effiecient imo.
Now since raiding is pretty much dead I think I'd go fvs unless I wanted a past life.