I like Drew better. He had a better year in 2008, plays for a better team with better players around him, is not coming off injuries, and could possibly bat in the 3 hole in 2009.
That said, I really believe Tulo is close and would keep him in your situation. Consensus has Drew as the #4 shortstop (behind Hanley, Reyes, and Rollins), and Drew could land you a much better SP if packaged with someone else.
i'd just toss them both around and see what kind of response you get. i definitely like drew more for next year but i can see what people are saying about the difference in value they bring back being bigger than the actual difference in value between the two.
I have Drew as #4 and Tulo as #5, so they are close. Drew should hit for more power than Tulo, even though Tulo plays half his games in Coors. That should tell you something.
I would shop Tulo and see what you can get. There seem to be a lot of believers out there as I have seen him taken over Drew in some mocks.