I am biased, but my vote is firmly with Thinkpads.
If I had to give a reason, besides the great build quality, it's the stellar warranty and customer service. Twice I've had to send my laptops in, and twice they came back to me in a matter of DAYS, fixed. They overnight everything, every way.
I wish I could say the same for Dell, but I can't. In fact, what pisses me fof the most about Dell is how they refused to take back THEIR OWN LAPTOPS due to some fruad thing going on. Was on the phone with 9 different departments for over 3 hours and no one would do anything. They finally "transferred" me to a number who according to the guy that picked up, was in no way affiliated with Dell whatsoever. After that, I decided we had 2 new free laptops (Dell reversed/nulled the charges because the purchase of the 2 laptops were made fraudulently.)