Which to Upgrade First?

[ardikus]

The most significant upgrades for me at the moment - a level 46 Monk - are the following:

Thick Banded Belt to Flowing Black Silk Sash

OR

Staff of Battle to Imbued Fighters Staff

Both upgrades cost about 10k - 12k, FBSS perhaps being slightly more expensive than IFS but a bit more common.

FBSS would effectively make my Staff of Battle 31/30 from 31/37, but of course I would lose the 8 ac 15 str from TBB.

I'm going to get both of these items eventually, but my question is: Which is more of an upgrade/which should I purchase first?