Unlocked doesn't mean you get updates from Nokia. That's unbranded. Unlocked simply means you can move from carrier to carrier. Nothing with updates.
If Cyan firmware is important, I'd wait until T-Mobile pushes the update. If you just want 8.1 OS, you can use the Developers Preview. The issue is, you cannot go from Dev Preview to Official Cyan + 8.1 yet, until Microsoft fixes the bug. You can downgrade and then get Cyan, but not everyone wants to deal with that.
Honestly, since you have a 1020, id wait for Cyan cause the camera improvements.