You live close enough, GO TO BOTH!
Not being a smart ass, just offerring an idea. You'll not want to be at the 100th Party more than a few days. The Black Hills/Sturgis adventure can be a few days OR as many days as you like. Thus, plan to spend a few days in the Sturgis area (rooms are cheaper during the last half of the event) and a few days in Milwaukee. As close as you are, you won't blow out too much vacation time.
Now, if for some reason you can't go to both, then IMHO, memories of your Black Hills/Sturgis Rally adventure will stay with you longer than a company sponsored "show", albeit a very significant show.
Now, if for some reason you can't go to both, then IMHO, memories of your Black Hills/Sturgis Rally adventure will stay with you longer than a company sponsored "show", albeit a very significant show.