First thing first, a 24 months contract is never worth it. You're better off paying part of the phone and get a shorter contract.
That aside I don't think any of the new phones is worth an upgrade unless you're desperate for a front facing camera, in which case Nokia is out of the equation. All the first gen phones run Mango well, there are no apps or games that necessitate a faster processor, the cameras are fine, really I can't see a valid reason to upgrade. Only larger internal storage would be worth an upgrade and it can now only come from the Focus S which I doubt very much will happen.
Keep your upgrade option in the air for when a proper phone arrives, I know it's hard because there's that desire to get something new and shiny but all thing considered it's not worth it, especially on a 24 months contract.