Disclaimer: I am not the absolute best cyclist, but I have turned in some okay IM bike splits. People’s mileage may vary on the opinion below.
I lived about two miles from the bike course and frequently rode much of the course during regular training rides even before IM came to town. As others can attest the terrain around the IM Wisconsin course is quite similar to that of the race route.
That being said, as others have pointed out, the course consists, with few exceptions of the following
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a number of flat or rolling sections (picture the “string” out of Madison to Verona, about half of the ride from Verona to Mount Horeb, as well as a few sections from Mount Horeb to Cross Plains and much of the road from Cross Plains back to Verona)
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a number of shallower climbs that are a bit longer in length (picture climb into Mount Horeb, a few climbs between Mount Horeb and Cross Plains, the long climb between Cross Plains and Verona just before “the wall” and Old Middleton Rd (?) just as you turn off of Timber Lane towards Verona)
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a few short “violent” climbs (picture hill just outside of Verona, multiple climbs between Mount Horeb and Cross Plains and of course “the wall” on Timber Lane between Cross Plains and Verona)
Considering these three different sections, #1 describes the longest section of road in terms of mileage, perhaps as high at 40-50% of the route. I think the #2 sections may account for about another 30-40%, while the #3 sections account for the remaining 10-30%.
So, which setup to use? Personally, I would lean towards a full tri setup. Most of my logic has to do with how I like to climb. I’m either in the saddle and in the aerobars (perhaps choking up) or I’m out of the saddle. I do from time to time ride seated and on the flats, but that’s rare for me.
That being said, I come out ahead in the #1 sections and can get through the #2 sections okay and regardless of my setup, I’ll be close to the same speed in the #3 sections (and about as "un"comfortable in both).
With a road bike, I would be slower in the #1 sections, slightly faster in the #2 sections and about the same, perhaps slightly faster in the #3 sections.
Now, if I were the type of climber who preferred staying seating more, and rode in the flats more often then getting out of the saddle, my approach would be different.
Under that scenario, I’d still be slower in the #1 sections, I’d be faster with a road bike in the #2 sections and #3 again really wouldn’t matter much. With a more seated style of climbing with a tri bike, the trade off would again be between the #1 and #2 sections (#1 being faster, #2 being slower).
I guess what it gets down to for me is how you climb/handle your tri bike. Because I can climb about as well with it as I do with a road bike, there’s not much of a decision to make. If, however, it was difficult for me to get up hills on my tri setup, especially in the shallow to moderate climbs, then I’d be further ahead with a road bike.
As for wheel choice, I think that also comes down to a personal preference and comfort issue. I ride on Deep V Zipps and use them for pretty much every race terrain. If I had a disk, I would probably ride that for everything. The only portions of the IM WI course I’d be hesitant on would be the relatively technical decent between Mount Horeb and Cross Plains and perhaps “the wall” on Timber Lane.
In the end, I think it depends mostly on your climbing style and how comfortable you are on your tri bike. If I were you, I would make up my decision fairly soon, and stick with it. As the race gets closer the last thing you want to be thinking about is bike choice and whether you’re going with the right setup. Been there, done that, and hope not to make that mistake again.
I hope this helps some… and good luck at IM WI.