It all comes down to the stats...how does it convert and retain. I like to try both options and see which makes me more money. I've worked with some programs on rev share that just keep sending me checks, month after month. It's great if you can build a recurring income.
Overall, i like to have a mix of both. I agree with Brian, it takes quite a bit to offer flat payouts, mainly a lot of cash-on-hand. Many programs simply don't have that. As long as they convert well and retain, i have no problem with it.