I don't think that it should be quality or quantity. You will do best if you can combine the both of them, checkout imlive.com and you will see what I mean.
We have been working on this site for over 18 months before we have uploaded it. and now we have over 7k active girls online, over 400 girls online 24/7 and each of them is a million $ girl
I think that if you can combine quality and quantity, you are sure to get the best out of the retention. If you are only working on one of them, you have a good chance to do well, but not to be the best. Just my 2 cents.