I'll bite- what am I missing?
FVS has more HP's ( not a alot, but 20 give or take assuming same starting Con)
* PM Could switch to Wraith, but then FVS is up about 60 HP's.
** And of course Yugo pots Con pots switch the HP advantage to PM if it's in Lich form, unless the FVS is WF
FVS has more healing options with quicker cooldowns that heal for more (unless a WF FVS no healing amp)
FVS has more DR
FVS has (generally speaking) more SP's
Pretty much every piece of useful tanking gear is usable by both, so no advantage there.
So, from my viewpoint- FVS is a better choice.
But again- curious if I've missed something obvious.