Pay Per Join is pretty cool. Ya simply can't complain about a big chunk of cash for a trial or one time sign up. Get your money from your traffic and be done with it and happy. hehe.
Although with some sponsors I simply don't expect them to have the ability to do that - smaller sites that are more niche oriented, newer, or more "mom and pop" - and am happy enough to participate in a partnership model. Especially if they rebill well, which really all of the ones I use do well. Partnership is a rational model, I feel, and I'm more than happy to participate in it on sponsors with whom it works.

In essence, I guess, I'm perfectly fine with both as I understand the function of each.

PS - CuriousCash, formerly BoyBucks, is one of my favorite programs out there