Doesn't matter whether you think Ward's a #2 WR or not, Mike Tomlin/Bruce Arians do. Ward probably won't be benched for either of those guys any time soon unless he physically falls apart(which may happen, but that's what it'll take). Ward actually looked like he was moving around pretty well after he got back from the thumb injury this pre-season. I wouldn't be shoveling dirt on him yet.
Even if he is moved to #3 on the depth chart and runs the slot routes(which he actually did a good bit last year anyway), Ward's still going to be Roethlisberger's hot read most of the time and he's going to be his WR target of choice in the red zone because of the trust they've built up.
As to the original question: Probably Brown for the first few weeks, but I'm of the mind that Sanders will steadily start to see more time as he proves that his foot is 100%. As mentioned, he's better at just about everything over Brown other than running the 9 route(That's not to say Brown sucks. He doesn't). I think both mitigate each others' prospects for 2011, but if either gets hurt the other becomes considerably more valuable immediately.
Cotchery clouds things a little for both these guys too, really, although I think he's more "Ward insurance" for 2012 and beyond.
Gun to my head I'd probably put chips on Sanders and hope he remains healthy. Brown may have more big plays by season's end, and do some more damage early, but I can see Sanders with better year-end totals if each guy plays the same amount of games. Sanders consistently just gets open.