If you don't talk much, T-Mobile's $30 plan is a great value. The plan only has 100 minutes of talk but has unlimited text and data, with 5 GB of data before throttling. T-Mobile prepaid also has voice roaming (unfortunately, no data roaming on prepaid). You can also use Google Voice and one of the IP calling apps (GrooveIP or Talkatone, for two) to help you get more than 100 minutes.
To get the $30 plan, if you don't buy a boxed prepaid T-Mobile phone, order a
prepaid SIM Kit from T-Mobile and then activate it online. As you appear to know, the beauty of the GSM sims is that you can move them from phone to phone, since it is the SIM that is activated and not the phone.
And, as was mentioned, if you need even more data, T-Mobile has a $70 plan (either prepaid or postpaid, no contract) that is unlimited everything with no throttling.
If you want an "unlimited" plan (unlimited voice and text but throttled data), I would suggest Solavei. While it is $49, it has 4 GB of data before throttling and both voice and data roaming.
Also, as was mentioned, AT&T has better coverage but slower speeds (not counting LTE). T-Mobile uses HSPA+ 42 (speeds theoretically up to 42 Mbps) while AT&T's network only supports HSPA+ 21.
You'll likely want to stay away from Go Smart, as throttle all data. The Go Smart $35 plan throttles all data on the plan to 2G speeds, their $45 plan has 5 GB or "3G" speeds (throttled to under 2 Mbps).
Net 10, Straight Talk, and Simple Mobile are all subsidiaries of Tracfone; their "unlimited data" is throttled (or even cancelled) but there is no set cut off. Some people have claimed to have no issues, others have claimed they have had their accounts cancelled for using 2 GB or data or less, with "excessive usage" being the reason given. If you use lots of data, I would stay away from them.
H20's cheap plans have very low data allotments. Even the $60 plan only has 2 GB data. The advantage is it uses AT&T's network. You might also look at
AIO Wireless, if you need AT&T for the coverage. AIO is owned (from what I've read) by AT&T and offers the 2GB plan for $55.
Hope this helps.