The best cell coverage domestically + sim swapping with t-mobile

Brooklyn to Portland by car

In August, my fiancee and I drove from New York to Oregon, clear across the United States.

In the car with us, we had 2 Verizon iPhone 5s, a T-mobile iPad Air and an AT&T Unite Mobile Hotspot. We took turns driving while the other person worked on their laptop in the passenger seat.

We also streamed music for a lot of the way on Pandora or Spotify using one of the iPhones.

Now, this is totally anecdotal evidence, but my experience with each provider was as follows:

Verizon worked everywhere with the exception of little 5-10 minute blips throughout the country. The only place verizon didn't work for an extended period was between Montana and Idaho.
AT&T worked almost everywhere Verizon did. Mostly though Verizon's LTE coverage was better than AT&T and the iPhone had faster data in more places than the AT&T Unite.
T-mobile worked well in big cities and sometimes worked while driving, but for the most part had poor coverage.

The 2014 ratings from JD Power seem to confirm our experience.

International Travel

This is a totally different story.

Verizon doesn't work internationally (CDMA is US only)
AT&T screws you on roaming
T-Mobile is awesome and gives you free roaming data (at a slower speed).

On a recent international trip I popped the Verizon sim card out of my iPhone and put the T-mobile one in. It works great when you arrive at the airport and need to get google maps, send an email, or get in touch with someone over chat.

The best part is you can do this on a basic T-mobile data plan for the iPad -- a 1gb plan costs $20 a month. I've used T-mobile free roaming in England and Mexico and if you're there for a short time and don't need voice, you don't need to get a local sim card.

So, long story short -- if you want the most reliable cell phone experience across the US, get Verizon. BUT if you're getting a second device, pick up T-mobile and you have an awesome way to get international data without surcharges.

P.S. It looks like the Verizon iPhone 6 supports everything the unlocked GSM model does + CDMA? Does anyone know if that's accurate? I'm looking forward to continuing to sim swap when traveling with an iPhone 6!