What is the difference between 89-00 seats and 01+ seats?

The newer seats are wider, more rounded, and lack the little "nose" that goes up the tank back.

Can't speak to comfort, I've only ridden the earlier bikes.  I have done 1000 mile days on my GS no problem.  I get tired, but I've never gotten a sore butt from it.

YMMV   

Well, I'd love to have a Corbin, but they only make them for 2000 and prior bikes. I suggested they make a seat for the 01+

I'm assuming the 01-03 and 04+ are the same, correct?

My butt hasn't gone numb yet, but I also haven't tried riding for more than 3 hours...one day, I'm going to have to do 8+ to make it to N.C. on the bike.

For a better fit, I believe you need an 01+ seat if you're going to run an 01+ tank on a '00 or older bike as well.