Right now, the C's have 3 rotation quality big men. They have 2 rotation quality wings. You can more than make due with 3 big men in the playoffs, you cannot with only 2 wings. So I think wing is the bigger need right now.
And looking at the vet minimum free agent options, I think there are much better options as a 4th big man than as a 3rd wing. Guys like Kurt Thomas, Kwame Brown, Juwan Howard, Craig Smith, Josh Boone, Rasho Nesterovic, or Fabricio Oberto would be better options as 4th big man then any of the vet free agent options would be as 3rd wing.
I disagree about the scarcity and need for this team. I have no doubt we could come out with a Posey/Daniels caliber 3rd (and possibly) 4th wing before the season begins, and I'd be conten with that. And of the players on your list, I'm actually fine with a Rasho or a Kwame in that slot (centers that can and have played starter minutes and can contribute).
But the overarching lesson I got from the way last year played out is that this team absolutely has to have length up front. That game 7 loss was more about the fact that Baby just isn't long enough to compete with the Lakers frontline than anything else. When KG and Sheed were in the game, we were ok. But when they got in foul trouble, it was a problem. Similarly, this year, in order to beat the Heat we need our big man advantage.
For this Celtic team, my hierarchy of necessity would be:
1) Big man defense
2) Effective initiation of the offense
3) Wing scoring
If any of our main guys get injured it hurts. But I don't buy the 3 bigs vs 2 wings on roster argument, because we really only have 2 bigs that can play the way we need to for the team to be at it's best. If either KG or O'Neal go down for any extended period, we need another 7-footer on hand to keep our defense tight. If that's a Rasho-type, that's fine. But to me it's a bigger issue than whoever we could get to back up Paul and Ray.
With healthy bigs, healthy Rondo/Nate, and either a healthy Pierce or Allen along with a couple of other replacement-caliber wings they could still play Celtics basketball at a near championship level. With healthy Rondo/Nate, healthy wings, and only one starting caliber 7-footer on the roster this team would be in a world of hurt as the defense is still their bread and butter.