"we hate paying for limited data"...shocking! most of us get a thrill out of it.
Seriously, without knowing your location, we won't know if you even have coverage on either one. T-Mobile has way faster 3G speeds, and blazing fast 4G. But their coverage is much more limited.
Spring is more widespread, but their speeds are significantly lower. Their 4G is almost non-existent except for large cities, and their LTE although fast, is also not widespread.
Based on that alone, I would recommend T-Mobile. But you need to know where you will be using your phone, and if it has the coverage you're looking for. Just because you have coverage, doesn't mean you have 3G/4G coverage, and just because you have 3g/4g coverage doesn't mean it's at it's full capacity.