Personally, I made the switch from AT&T to a pay-as-you-go carrier in the fall of 2014 shortly after the iPhone 6 came out. Contrary to what you might assume they offered financing, and after paying it off entirely my bill is only $30 a month. Which includes talk, text, and data. The best part is there’s absolutely no contract whatsoever.

Enough about me, should you switch cellphone carriers as well? We’ll do our best to come up with a reasonable answer based on what you’re paying now, what you are looking for in terms of services offered, and generally that sort of thing.

What's Your Current Plan Costing A Month?

Is iPhone Availability Important To You?

Do You Want Free Phone Upgrades?

How Important Is Low Month-To-Month Cost?

Since upgrade availability is important you I'd say sticking to the major carriers like AT&T or Sprint is you best bet.
Pay-as-you-go cell phone services are cheap because they rent the excess bandwidth from major carriers like AT&T.
Even though its a month-to-month carrier Consumer Cellular financed my iPhone 6 for just $25/month and no added interest.