Besides the keyboard, and an extra couple of megapixels, and built in memory (surely a 32gb MicroSD would be cheaper?) I can't see anything to make the N97 better than the 5800.
It looks like it'll have the same screen, same processor, same features,
Speaking of competitors, If I was really interested in the 5800 I would try and hold off until the Samsung i8910 Omnia HD came out. It's suppose to be this quarter. It's not going to be $200, but it's staying power might make it a better deal in the long run. And it will almost certainly be a better experience.