I don't have any direct experience with those two, but I own several Dell computers and a Thinkpad T23.
All of my Dell computers have had serious problems. I've had two Optiplex GX-1s that totally ceased to function, a Dimension 2100 that had major issues with sound card compatability, and a Dimension 4300 motherboard die on me 6 months out of warranty. $150 to replace and it wasn't even the same model. Oh, that and my friend bought a Dimension XPS with a Radeon 9800. The card had all sorts of issues and eventually stopped working at all, he had to replace it. I personally will never buy a Dell product again, ever. I will do my best not to use a Dell product again.
Whereas my Thinkpad T23 has been abused more than all of those systems put together. Had it, running fine, a cousin spilled tons of soda on it while it was running. It freaked out and stopped booting up. With very little knowledge of laptop assembly, I completely took it apart and washed a few parts in the sink. I let it dry for awhile and when I put it back together, BAM, it worked 100% fine.