I've not seen any data that supports one over the other as far as success of grabbed names goes. Do you have experience that shows something different?
Thewitt, have you used either service?
I have tracked names on many an occasion. I always look at who gets the names that I am after and the data points to snapnames being pretty good at what they do. expirefish does not even come close. It is always changing, companies evolve different techniques etc and who is king of the domain name expiring market one day may not be the next. IAREgistry used to be one of the top players, not any more.
I have also used namewinner and snapnames on the same name and snapnames wins out.
Of course snapnames are far from being a successful gurantee of securing an expired domain name, there are people out there who must have links or deals with some of the smaller registrars who repeatedly beat snapnames every time for the very top names.
But I dont think you will find any data to dispute my claim that snapnames is better than expirefish, if you do, please let me know.