They definitely stay with sites for long periods. Look at how the site owner makes some of his money, (besides upsells). In a pps model, they pay the affiliate basically the whole first months worth of revenue. (usually about $25 to $35). The site owner is counting on his members area being good enough to at least retain that customer for another month. (Or the customer just doesn't realize he's going to be re-billed).
With rev-share, it's typically newer sites or sites in a specific niche that work well. Solo girl sites and really weird niches tend to do well on rev-share whereas a "big tit" site may not.
The best way is to do a split test yourself, (if they offer both).
I typically use pps because I want my money up front and dont' like being seeing sales disapper if the customer does a charge back.