Data in the US is more expensive than ever, even so the caps will kick in to throttle traffic.
For US, it's either T-Mobile or AT&T.
Roam Mobility idea is good, but the plans are expensive if you compare the day plan offered by T-Mobile. Especially when you cannot roll over the unused days. I pay $2 or $3 a day for unlimited talk, text and data with T-Mobile and my account is good for 1 year with a min $10 top up a year.