I think it will help to know what you intend to do with the N810. Just browsing? email? Media player? It is because an N95 or a nokia E series phone (they have one with a keyboard) can do those things too. If the smaller screen does not bother you. (check it out on the store). S60 phones also have a lot of software add-ons too(commercial and free). But if you intend to learn Linux or really want a mini-computer then that is different. Experience in web browsing on the N810 is a lot better though (screen and browser) but the S60 phones are not bad.

I choose to have 2 devices path for myself with N76 phone (due to smaller size) and a N810. Like the other posters said they do compliment each other.