Solavei is best $49 for 4gig of true 4G data. I get usually 10-15mbit. Also Solavei is only MVNO with full data roaming (not just voice roaming). I roamed on AT&T HSPA and actually got 4mbit speed. I don't even think T-Mobile direct post-paid gets that.
I was actually just looking at their TOS. They actually ALLOW tethering. Impressive. I didn't realize they roam on AT&T until you mentioned it. It's the best deal by far really. My mom will be switching to it in a couple of weeks.
I'm on the $30 T-Mobile plan which is still cheaper for me for the time being since I don't use more than 200min overage each month. I may go over by 100min at the most which makes my total bill $40.