Sounds like the National 24-Hour Challenge.
There will be folks riding rear disks, but there won't be many. Among the strongest riders out there, I recall the Guths rode them in 2005, but not last year. I don't recall any of the other contenders riding disc wheels. You'll definitely see lots of aero wheelsets out there. You'll even see a few folks try it on TT bikes (although none of the big mileage folks will be doing that). I've never seen anyone riding with an aero helmet at Michigan or in any other Ultra event (but then I confess I wasn't on the lookout for it, either). Had anyone tried that last year at the N24HC, I bet they'd have DNF'd on the 127-mile day loop. The temps were in the mid-90s with extreme humidity on that loop (the start temp was 82F!). The casualties on that loop were positively ugly.
I'd go with the equipment that you're the most comfortable with, whatever that is. Any advantages from the most aero equipment will be radically offset by a comfort disadvantage, if that's not the gear that you're most comfortable using. One of the truths (that many don't like to admit) is that for the really long stuff, the gear just doesn't matter all that much. What matters most is that it fits you and that you're comfortable using it.
That said, if you're still game, bring the disc wheel. There will be lots of folks still averaging in the low-20s into the night. You'll be surprised at how high the speeds at this event are. I finished the day loop with the leaders last year in just over 6 hours, and that was considerably slower than in 2005. The speeds are high enough to justify it, and the course has little climbing on it. Once you get that puppy rolling, you're not paying any real performance penalty, and the aero advantage, especially in a crosswind, is considerable over that amount of time. If you're consistently fast enough.... Personally, I'd leave the aero helmet at home, but YMMV. Good luck and see ya there!