Between the two, I definitely use Ancestry the most. While I think most people really love them for the census records, I think I like them more for some of the vital record information and images they have in their databases. Yes, that information is a bit spotty depending on what state your are researching but it has allowed me to obtain this information / documents without always having to request these document(s) from individual state's vital records departments, thus saving me both time and money.
Even though I've had a subscription on Fold3 / Footnotes for a few years now, I've not really used it that much but, and this might be strange, but I have always liked it. Fold3's main emphasis these days is on military records. That being said it still has other records on the site from its Footnotes days such as the African American Collection and Native American Collection. Since focusing primarily on military records now, it's might be a bit harder to actually find these collections but they are still there.
In the past, I've had a few other paid online tools and can't even remember which ones they are now, which I guess attest the the fact of how much I used them. Needless to say I allowed them to expire due to lack of use.