I've picked Dott, because Dott was a World Champion, and Stevens wasn't. But I think it is perhaps a more difficult question then it originally seems.
You kind of forget how good Matthew Stevens was. He was the UK champion, The Masters champion and has reached The Crucible final more then once. But all he generally gets remembered for his that final when he lost to a qualifier. And, you must admit, Murphy has gone on to become one of the top players. Winning also the UK championship, don't forget.
What seperates them though, is that Dott tanked down the rankings when he had personal problems, and managed to get it back together and climb the rankings again. And has reached a Crucible final since then. I think Stevens has never recovered from the poor run of form that got him out of the top-16. Players like Dott prove that dropping out of the elite is not the end of your career, it can be only temporary, even under the old ranking sysem. Matthew Stevens may be the more talented of the two, but Dott is better at the mental side of it, and keeps it together to make the most of what he's got perhaps. I think somebody who was UK Champion, and Masters Champion will always be disappointed that they never won the big one. Especially when, like Stevens, you were in two finals.