In terms of what the differences are, it seems that there is no difference for the program for either its post-paid or prepaid customers, although AT&T notes that prepaid customers on a $35 or $45 monthly plan will be automatically opted in the Sponsored Data program. Other than that it is expected to function pretty much the same way.
That being said, this raises some questions with regards to net neutrality, in which a couple of years ago while the rules were in effect, the FCC expressed their concern over such programs in which they felt that it was a violation of net neutrality. Now that the rules have been repealed, we guess it might not matter as much anymore, although AT&T has reassured customers that nothing will change regardless of the outcome.
However with several states introducing their own net neutrality rules, we have to wonder how AT&T plans on implementing these programs without running afoul of said rules.