Best Shorts for an IM

It's whatever YOU feel comfortable in.  Are you shooting for time, then don't change.  If you feel more comfortable, then change.

The tri-shorts give some compression, though.  Just a thought.

I wore tri shorts on the swim, then put bike shorts on over my tri shorts. Then at T2, i just stripped off the bike shorts and ran in the tri shorts. Of course, try this out in training before the race. I wore the same top throughout the race.

Oomph Lava or Kilauea tri shorts are great for IM - I like the Distance Pad those 2 models have for longer stuff.  Wear them for the entire race swim thru run

Most people really like the Oomph shorts.  I absolutely hated them and gave mine away.  My favorites are the Sugoi RS bike shorts for long rides and Sugoi RS tri shorts for tri's and the shorter stuff where there is no area to change clothes.

I switched into Bibs for the ride and regular running shorts for the run.  Why bibs?  I wanted to make sure I didn't have a gap along the small of my back to be sunburned.  Opted for a full-zip jersey to make restroom brakes manageable.  Note that I'm not gunning for a Kona slot, so comfort was far more important than a few seconds not changing or peeing on the bike/run. 

The best shorts are the ones you trained in and are comfortable with. Train how you plan to race. I only train in tri-shorts, and only race in tr-shorts. Many 100+ mile rides in them and I have no problems.