Can't compare? Well partly that's true because higher end Dells use this years graphics versus the last year graphics installed in the latest and "greatest" thinkpads.
But even if you discount the graphics at least some review sites think they compare.
CNET sucks but even if 1/2 of what they say is lies, there has to be a grain of truth in there somewhere:
Stunning 6.3 out of 10
http://reviews.cnet.com/IBM_ThinkPad_T43/4505-3121_7-31312127-2.html?tag=glance
Comparable Dell 8.0 out of 10
http://reviews.cnet.com/Dell_Latitude_D610/4505-3121_7-31268759.html
IBM laptops have the same reliability as dell (but better service when it breaks):
http://www.pcworld.com/reviews/article/0,aid,118514,pg,5,00.asp
If cost is no object obviously go with the T43. You'll see no diference in performance with your apps but you will be so happy when you say T43 instead of T42.
If you are maximizing bang for your buck get the T42 and spend the $500 on a docking station and 19" LCD. You'll get tons' better computing experence at home, and better battery life on the road.