You're not necessarily better off getting $40 per paid trial than you are getting $50 per active, simply because free trials convert a fuck of a lot better than paid trials, and the word FREE in your text link will get you more clicks.
(The only time I think the "per active" model is a bad idea is when its for a paid trial, if you're gonna give me $35 per trial or $50 per active on a paid trial, well DUH!)
Just like any other sponsor, throw traffic at different programs and compare results. I have friends who swear by certain sponsors that I couldn't sell to save my life, and vice versa. There's no hard and fast rules about which program is "the best"
As for FM's proggie, his stats are a bit of a pain in the ass, but they are very detailed, and the checks are always here on time
