Awesome news!
Last promo was great - albeit quite a pity that not too many converted to paying members for me personally. But you guys paid for about 70% of my new car, so much love to IWantU!
I'd say fiddy+ bucks per sign up is best for both yourselves and affiliates. That way you'd eliminate freeloader traffic and it'd benefit current loyal webmasters with free members (who are ready to whip out their credit cards).
Downside for people who jump the ship will be that on average it takes 3-4 weeks for your average free member to sign up. So they'll get the fullest out of the first week of promotion, and will get paid regular rates towards the end of the promo.