Depands on your site's traffic actually and the niche, and also if you have more merchants who offer good CPA or more merchants who offer good CPL for the niche your site is all about.
Why don't you test pay per lead and pay per sale and see which converts the best for you. I must admit that pay per lead is likely to be easier, but the earnings per lead would normally not be as high as what you would get per sale. So ROI is what you are looking for here.
There can be some vey hih commissions on a per lead basis -check out shareasale, there is comisson of up to $150 per lead!
When it comes to relevant ads, always try to get the most reputed and highest rate for pay per lead and then come up with pay per sale and then calculate the differences. i.e when it comes to dating i would take a high pay per lead.