Apparently AT&T's old unlimited data isn't really unlimited if you are having overages. We are grandfathered in on Sprints unlimited data and it is unlimited. I can stream 24/7 for a month and pay the same bill every time. When we upgrade our phones they try to convince us the new plans are better. Wrong, when I contacted a rep she told me to never get rid of our plan because it will never be offered again. I think we started this plan back in '04.
I'd do whatever saves you the most money.