I think I might go for the sword for 5v5, in 2v2 and 3v3 the stuns are more valueable. But for 5v5 the extra damage might be even better, it kinda depends on your setup I guess.
Seeing as you run a doubble melee with WF totem the double Sword Specc hit and the Windfury proccs adds up. But so does the stuns from mace specc and Stunherald.
Stunherald might be better when you are nuking for instance a 4 dps team, but then again I believe sword would have an edge versus 3 healer.
You can of course try it out and see what you enjoy the most, unless the warrior can spend his points on better stuff
If he still lacks other pieces of s3 gear, I'd get them, and keep the mace, as it is not bad at all.