Definitely Derek! Derrick made me automatically think of an oil derrick. I don't remember ever seeing Derrick as the spelling for the name. I've seen it spelled Derek many times, and Darek and Derik.
I've never seen it spelled Derek (except for Jeter, of course)! I've always seen it Derrick. Of course I'm from Texas and there are oil derricks all over the place, so maybe that's why!