If you want a SMARTPHONE, you won't be dissapointed with the N9. My friend has SGS3, and he said if there was bigger version on the N9 (like 4.5-5'') he would pick it over SGS any time of the day. So I guess this says something about the Nokia.
However, if you want MOBILE PC, you should stick with N900.

I personally like the N9 way more than the N900, but that's just my opinion.