Fleury, no question. He's had some bad stretches, but also some very good ones.. He was a big part of that cup team, and thats something Rask can't say.
Rask definitely has the potential to grow into a better goalie, but you know what you're getting with Fleury. I'll take the proven vet with much more experience, instead of a 'maybe'....
Fleury has actually never been that good.
In the 2012-2013 Hockey Prospectus preview (Penguins team page), there's a pretty good explanation of how he has been below average throughout his career. Not sure if I can copy and paste it because this board has silly rules on that stuff, but on the 4th page and on the last page is where you'll find the info.