[mattnewton]

Well for one thing the first is not a great business model compared to the second. (And I suspect that's the primary consideration)

But, you probably mean from a user's privacy perspective- and then I'd still say the answer is yes. The companies buying ad exposure in the second can't easily re-sell your data to another third party, and they can be constrained from what they learn about you. The first example basically means you have to trust the whole world and the second example means you have to trust the primary data holder (a lot).