It's a tough decision to make isn't it? So many variables. I've contented myself with splitting it. The sites I think will retain the players due to their quality, I take the rev share. The others, I do the single payment. I suppose mix it up and put all the eggs in one basket is the answer.
I think it depends where you are with your site as well. If your just starting out then getting the cash straight away is important but if you have a more established site then it make sense to think longer term and go for the revenue share.
Personally I find it hard to turn down money now on the promise of cash in the future!!