Tri or Rode Bike during long rides

I'll be riding my tri bike at IMLP which is hilly.

I'm more comfortable riding in aero even uphill. Most rides I ride 98-99% of the time in aero position. I am less tired doing a 3-5 hour ride on my tri bike as my upper body is supported by my skeleton system as I ride in aero bars. When riding my road bike my arms support my upper body.

Look at most IM races the majority of folks ride tri bikes but there are road bike as well.

I use my road bike for riding in the winter, bike racing, but not ever for tri racing. I'm faster on my tri bike as well as I'm in a more aero position. 

Whatever bike you plan on racing typically.

Early in the season I might mix it up and ride on my road bike for some and tri bike for others to adapt different muscle groups.  But as the races get closer it is all tri bike pretty much.

I first answered this with "I train like I race - on a tri bike" but then I re-read the OP's question more clearly.

A tri bike is designed to ride in an aero position thus saving the rider energy/time and therefore its greater benefit would be realized on a longer course vs a road bike. Given, more benefit is gained from riding on a flatter course vs hills because you can remain in an aero position for longer periods and many would argue a road bike is better suited for hills.

As for a road bike being better for longer rides, I could see from the perspective that you may be more comfortable on a road bike for a long rides, particularly training rides, but I would also argue that a well-fitted tri bike can be as comfortable as a roadie.

Is your road bike a good bike for an IM? It is if it's the only bike you have!  Should you get a tri bike for an IM? Ask yourself if you'd be more comfortable on a road bike or a tri bike, then ask if the course would be one where you would spend most of your time in aero, then ask if investing the money into a new tri bike is worth it to you.