My contract is up with T-Mobile (UK) at the end of the month and I'm thinking of just keeping my Omnia 7.
The HTC 8S looks nice but it's pretty low end and doesn't offer me anything new. The 920 is still really expensive for what it is (you can get an iPhone 5 for less). I'd would probably get a Nexus 4 but they don't offer it.
tl;dr smartphones are boring.
If you are still okay with the Omnia just keep it as long as possible. New phones will only get better. Also, the current 24-month upgrade cycles for phones (encouraged through hidden costs in contracts) are too short. I don't think I'm going to replace my Optimus 7 anytime soon. Still works great.
That said, on the low end the Lumia 520 and 620 are pretty great. If I needed a phone now, I'd probably get one of those.