My experience:

Verizon has by far the best coverage and quality, but is very expensive. International data is an extra charge, either by the horrendously expensive minute or the marginally expensive add-on plan. If money is not an issue, Verizon would be the choice.

T-Mobile is least expensive. It works pretty well in Europe and has (slowed down, but useable) unlimited international data included. Coverage in the US is spotty and unreliable, though.

Don't know about ATT. I got in a whizzing contest with them years back and have not voluntarily given them a dime since.

Sprint has the same deal as T-Mobile with international data -free and slowed down - but has better coverage in the states. Its cost falls between Verizon and T-Mobile.

We switched to Sprint a few months ago from Verizon. While the quality of Verizon is noticeably better, Sprint is certainly adequate both at home and abroad and about $50/month less.

The slowed down data overseas is noticeable, but is still fast enough for voip calls and light surfing.

As always, YMMV