Well, that depends.. i mean what are the specs? Both can come customized so you'd need to give more information.
If you're talking both with the same specs.. i'd get the thinkpad.. i love them because they're built rock-solid. I had a thinkpad before and it was by far my most favourite.. and since i currently have a dell.. that's relevant =)
That being said.. thinkpads are more expensive than dells.. but the quality is better if you can afford the difference.