Maybe you could use google voice with your wife? Do you have previous contracts that you want to keep (that would make the decision much easier) ?
I have a feeling you're at least a little tech-savvy and your wife is not. The n900 does have some advantages (like being able to run shell scripts or lots of available apps for anything) which would make it easier to customize by you for her, just the way she wants. Also, having the same phone, you could more easily fix problems with hers