Let's assume that all cam sites are giving you a fair shot for referral conversion.
We can go on for days arguing that some are not and never prove that point conclusively.

Then it comes down to the differences of the cam site;

1. the "premium" v. the "freemium" business model,
   
2. the referral payment model: PPL, PPS, or rev-share,
   
3. the website's customer appeal,
   
4. the cam site's credit policies, customer support,
   
5. the communication between the Models and the Customers.

All the rest is endless drama.
Find what works for you and do business where you believe you are treated fairly.