I think it is important to test your domains at a variety of parking programs as no provider will perform best for all of your domains. You will find that some do better with Parking company A, some names will perform better with Parking company B, and so on.
What I would advise, is to first run your portfolio at one provider for at least a month, this gives you a good base in which you can compare stats to. Then pick at least 5 different providers (a mix of both Yahoo and Google) and test all domains for a minimum 2 weeks.
After testing across the different providers, compare stats on a domain by domain basis. Then move domains to the provider that performed best for each.
Cheers,
Mike
Fabulous.com