That changes year by year. It's gone from no down payment and 1/24th of the total price plus interest each month (in 2004) with a 2 year lock-in, to buy your own phone, to down payments, back to no down payments - from a 1 year lock-in to a 2 year lock-in to a 1 year lock-in - every time you look one carrier or another "comes up with a better plan" (for "better" read "makes them more money") and the others soon follow. Worry about down the road, down the road. There's no way of predicting what they'll do next month.
You pick the carrier with the best coverage for the places you need coverage, then you either buy a phone outright, or get the best deal on a phone you can get from them. (And shop around - different independent "branded" stores may be willing to come further down in price than others, just to get your business. Or the one you've been dealing with for years may be willing to come down further to keep it - or to thank you for loyalty.) (We had one woman who made a 2 hour trip to the store because we were the only store that treated her honestly and respectfully. So I'd occasionally throw in things for free. She appreciated an honest businessman - I appreciated a loyal customer.)