1. One engine may have a performance advantage at one track, while another may have an advantage elsewhere.
2. Rebuild schedules? How hard to plan to push it? The Rotax is lower revving, so it should be able to last a bit longer.
3. Costs of rebuilds? Rotax has to go through a Rotax authorized dealer, and has to be sealed, so it’s more expensive.
4. Ease of tuning? Really depends on how you define “ease”. The Max is “easy” to get running, but in order to get maximum performance, you have to test to find what jet to run in what conditions, and constantly monitor air conditions. The IAME’s are also easy to get running, but are adjustable on the fly – this can be good or bad. The good is you can dial in your performance on the track. The bad is you can go too far and possibly seize an engine from going too lean.
5. Other factors:
a) The IAME’s perform more like a “real” kart engine. Most people find them more fun to drive.
b) Most importantly – who do you deal with (kart shop)? Unless you’re dead set on one engine over the other, your best bet is probably to go with whatever product you can get support for.