what bike for IM WI?

Tri bike all the way.

Tri Bike hands down. There are hills only...no mountain passes.

I am doing IMWI for the first time this year. I signed up for the racing owning a Serotta road bike. In the winter, I decided to get a Cervelo P2C. I didn't purchase the bike because I was doing IMWI. I bought it just cause I wanted a new bike and got a nice bonus.

I have ridden the IMWI course on both bikes and both worked well. I did my early season training on the road bike and after the Triple T switched to the tri bike.

While the course is somewhat hilly, there are large sections where you can stay aero for a long period of time (most of the way from Verona to Mt Horeb, after the rollers on the way to Cross Plaines, from Cross Plaines to the climb on Old Saulk Pass, and many others). And with the exception of the 3-5 climbs, most of the climbs you can actually stay in the aerobars if you are comfortable climbing in your aerobars.

But that doesn't mean a road bike won't work. I have volunteered at the race the last 5 years and I have seem plenty of road bikes on the course. In fact, at least 1 pro women that I can remember road her road bike with clipons.

I would say that bike fit and comfort is the most important thing out there.