As a mobile computer you will not get anything better than the n900. It is still the most powerful and best even though it has one year on it's neck. But you need to learn it. If you plan to dual.. then get the best phone out there, with plenty of battery life and not another smartphone.

If you want another smartphone.. then wait a little bit, like a few month, and get the best one. If Maemo wasn't for you then maybe Android is the way to go.

Edit: Ohh.. and put down how you use the n900. What you do most on it. The smartphone you will buy needs to do that and also be better at it.