I've used Gophone for years, from when they were actually about the only choice for prepaid, non-contract data (2009 on a Nokia 5800). I like the reliable service, I like the fact that customer service has actually always been good and pretty well informed and I like that I don't have to mess with settings... just pop in the SIM and go. I also like that you buy the data and it's yours. You can do with it pretty much anything you want, hotspot, whatever. Saving $15 a month just isn't enough motivation for me to switch, especially when I see so many people struggling with ST. The T-Mobile plan is great, but I don't get sufficient coverage (don't believe their coverage maps).
Definitely do research and get the best deal you can, but I'm happy with my AT&T $60 Gophone plan. If I need more data in a month (haven't ever yet) I can add 1Gb for $10.