There's not enough highway "room" in Maui to allow you to exercise most bikes to any extent, and police enforcement is strict, so the type of bike you get is not really critical. I would probably just go for a Sportster. You're a Beemer guy, so you will be wearing a fully face helmet, anyway, right?
I've rented Harleys, and they're lots of fun.
If I lived on the island, I don't think I would own anything much bigger than a 400.