We have all our phones (3 Samsung S4's and one iPhone) on Sprint with unlimited data. Phones work great and coverage is awesome because Sprint users roam on the Verizon network which is everywhere. The bad news: Verizon pushes Sprint data customers roaming on their network down to a 1XRTT connection which is almost unusable. Sprint is scrambling to build out their 4G offering but they lost a lot of time fiddling with Wimax 4G and are now behind the curve coverage wise.
So now we have a 4G mifi on Verizon, 5GB for $50/mo and $10 per GB for each one over 5. Look at teh coverage map, AT&T and Sprint don't even come close to Verizon. If you like your AT&T phones, keep them, just get a Verizon mifi. You don't have to be all or nothing with one carrier.