Its individual choice and depend on situation.
Maybe you get the wrong idea coz i am not supporting any of them (I don't have both of them and mostly hire from guild).
Perhaps give you a more detail info of what they can do and you can list out whichever you already achieve and compare which hero give you the most benefits.
Pandor
1)Majority players uses him in arena (Optional since I've gotten rank no.1 in arena without him, probably luck). Combo with Blake greatly improve Blake's survivability as Blake will rush in to absorb most of the damage. Pandor gets interrupt lesser while providing barrel toss to support blake.(hit rate increase and opponent dodge rate decrease).
2)One of hero needed for phoenix prophecy pool (if you are not using Shaka's combo)
3)The duration of his ulti stun lock an opponent is very long incredible effective in raid boss (against hyral and Capt'n). Combo with Noire, Sarpa and Leo can almost perma stun the boss in the entire battle if coordinate with right timing.
4) Dojo Physical (optional coz there are other substitute hero to replace him)
Vincent
1) Good in arena to combo with lucas (even better after awakening) for Arena defence team.
2) Magic dojo (same like pandor, optional)
3) Like i mention in last post, great in most of prophecy pool.
Eventually you will need most of the hero but you have to priority on who to up first due to limited of resources. Go for whoever you think is right for you as there is no correct answer for this.