i have owned both devices and the n900 wins hands down. speed is the main issue for me on the n97. it really struggles sometimes. the fm transmitter is much weaker on the n97. internet is only acceptable with opera browsers which struggle on ram. but it does have more complete and mature phone features.
its up to you but the n900 is better at everything they both do and the ui is nicer. and its open source, but then again u can play around with cooked firmwares with symbian now so still lots of modding fun.
like i said ive had both and i prefere the n900. hope this helps