N97 is Nokia's Big phone ... i.e. the competitor for the much compared iPhone. A device aimed at the masses, very low chance of confusing or alienating buyers.

N900 is there new internet tablet, with gsm functions. Not a "true" phone in that its feature set's not what the average phone user expects. A device aimed at very savvy users, very high chance of confusing or alienating average buyers. [See various useless flames and comparisons between the N900 Tablet and various phones].