Multistrike and Crit don't provide their full benefits in pvp. Multistrike only has half the benefit it does in pve.
Agi, stam, crit, mastery are same between the 2 comparisons. With off chest you get 6.12% multi, 2.57% haste. With off helm you get 1.67% multi and 5.51% haste.
Now we just need someone to do the math on 2.25 (4.45/2) multi vs 2.94 haste.
Edit: And if the damage is the difference of 0.1%, then i'd rather have the extra haste for more pred swiftness procs.