Why do you need/want an unlocked phone? Just curious. Do you plan on relocating to another area (different state or country) in the next two years? If not, ideally you should consider taking advantage of the subsidized pricing on these phones from T-Mobile, etc. A new, unlocked HTC One will be in the neighborhood of $650. The Sony you mention is ~$500.
Another thought. You could buy a subsidized T-Mobile phone, have it unlocked at a shop for $30-40 if you eventually think you will need to switch to another carrier. YMMV. Also, you could pay the ETF and get out of your contract by selling the phone outright too.
Not sure when a smaller city like St Louis will be getting LTE, so that may not even factor in for you in the near term anyway. A device which is capable of HSPA+/42 may suffice.