I argued on a big Tigers board early this year that Grady Sizemore was better than Curtis Granderson long-term.

Granderson is better.
Say what? Granderson is having a good season and is a very nice player, but let's not get carried away. You had it right the first time.
You really think Granderson will have a better career than Sizemore?
He's at least as good now. Granderson has better range in CF and maybe a little better arm which isn't saying much, and Granderson has made six errors in CF over the past 300 games, Sizemore 5.At the plate Sizemore has more HR power and draws more walks. Granderson has good gap power though and is probably a better baserunner after a hit. They both strike out a lot and they both have trouble against lefthanded pitchers. Sizemore is in his third full season and Granderson is in his second. Why are you so sure Sizemore will be so much better? He certainly is not a better fielder and he's just a slightly better hitter because of the walks. I can see the arguments for either player but I don't see a big difference either way. They are both good hitters, both good fielders, and they both are good baserunners. Sizemore is a year younger.
I think both will increase their power numbers as they get older and both could shift to LF.
I'd say the difference in defense is negligable. Sizemore is two years younger using "season age", as he just turned 25 last month and Granderson will be 27 before next season starts. Sizemore has shown pretty consistent OPS numbers over the past three seasons, whereas all of Granderson's MLB OPS success is based on his great '07 campaign.
I'd agree that using Granderson's 2007 campaign compared to the average production of Sizemore 2005-2007 grades them fairly equal, but I'll take the kid that just turned 25 and has been posting All-Star caliber numbers since age 22.
I'm not saying Granderson is a bad player, I think he is very good. I'm just not sure how anyone can say in good faith that he is better right now, or that he will have a better career than Grady Sizemore.