I would consider whether they are likely to be repeat buyers. If so, consider which agencies give you ongoing referrals without a limit.
ShutterStock will indeed pay you $50, which Bryan generously matched. However, Dreamstime pay 10% over three years, which is $50 now plus whatever they spend in purchases over the next three years.
If they each spend $500 every six months you've got yourself $600 over three years.
I have a table of referral programs similar to Steve's:
www.microstockdiaries.com/microstock-moonlighting-with-referral-programs.htmlMy personal referral strategy is to focus on referring contributors to ShutterStock and buyers to Dreamstime. They are the most lucrative programs long term.
...until the next change.