I'm not clear on what you mean with backup. Functionality or content, and what part of it. The N810 will work fine as a browser, it has the same Opera beta version that the N900 just got, and it has several of the other browsers that N900 can have. Both have pidgin and similar for IM, some ebook readers, music playback. Both has underwhelming navigation.

You can't have as many programs installed on the N810 as on the N900, it'll complain sooner. The same programs are often not available on both devices. The "same" programs are usually quite different in looks/menus/versions from the two devices. One of the things that struck me first with the N900 was that many programs I expected to be available, hadn't been ported from the N810. When they came, they were changed.

So, it's really not like having to computers with the same OS. It's more like Windows 95 vs Windows NT. Yes, in theory they're quite alike.