ThinkPads have a higher resale value as a rule. You can perceive that to be a result of higher quality/reputation, etc.... hence the higher value.
I would say the quality given that is higher for the ThinkPads.
IBM sells reconditioned TP's off their site, and I know a lot of catalogers do as well... Namely, TigerDirect.com. Compare the prices from both... TD also sells used Dells... so that may be another good way to compare perceived or real value of both.