2 piece bonus.

[bertzz]

25% damage to Ms and Bt
2 piece bonus is kinda more solid to arms since mortal strike hits harder than bt and has only 2 seconds more on cooldown.
maybe a 35% buff to bloodthirsty would be fine ;p

[Xtacle]

Well Fury does more damage than Arms anyways, so honestly I don't mind it being like this.

[Mataru]

BT also crits twice as often MS. When you put that together with a 25% shorter CD, BT doesn't come out too far behind MS.

Edit: Actually because of the high CS uptime of Arms MS does come out ahead by a pretty large margin. But Fury does do more damage overall so it doesn't really matter. Arms doesn't scale as well with gear so it's not so bad that they get a little better use of the 2-piece.