I'm having the same debate myself. I'll share what I'm thinking...
Myers had a monster rookie year and has settled now. He's still pretty solid but to expect 50+ year in year out is a stretch. With ownership in Buffalo changing and rumored to be willing to spend, I can't help but think that they'll focus on aquiring/signing a real puck moving defenseman. Myers will not be the focal offensive point from the D and I think that helps him develop into a really good all-around defenseman. Think 40-45, average +/- and solid PIMs.
Hedman is taking his time but looks to be taking the same development course, minus the monster rookie year. I think that Tampa will address the need for a puckmover as well in the offseason (how slick would Kaberle look there) and Hedman will the Pronger to whoever they gets Niedermayer (purely hypothetical of course).
Now, seeing as they'll be very similar IMO in all aspects, gimme the stronger team for my choice in a keeper and voila, Hedman it is.
Yzerman and Boucher, with Stamkos and Hedman growing together should make for some exciting hockey for years in Tampa. Buffalo I'm not so sold on...