Vitality until your healers know the fight and you are comftorable with busters (if you are new to tanking, you have not seen this. Neither current EX fight has one really. Prepare your bodies for this come Alexander, as its the staple of SE Raidis).

Once you are at that point, you can go strength.

Also I believe your MP returns are flat, not scaled off strength, unless you have blood weapon on (which you wont have on unless you are off tanking anyways). Your siphon blade restores a % of your MP (i think, 9%, same as riot blade).