For completeness, one FFP you didn't mention is Air Canada and their Aeroplan partner. You'd have to check earning potential, and they do have higher redemption surcharges, but the expiry policy is similar to UA. On the other hand, AC has recently announced an intent to separate from Aeroplan and launch their own FFP in a few years, so it is unclear where you'll be able to spend any Aeroplan-accumulated miles.
On the whole though, UA is probably your best bet.