Dell Precision Series M4700
Height: 1.29" to 1.44" / Width: 14.80" / Depth: 10.08" / "Target Starting weight": 6.33 lbs
Apple MacBook Pro 15"
Height: 0.71" / Width: 14.13" / Depth: 9.73" / Weight: 4.46 lbs
You can CL a used MBP for a considerable portion of it's purchase price 2-4 years later.
If you're unwilling to sell your old MBP, the Dell may very well be a better deal.
MBP: 2880x1800 Dell: 1920x1080
In the end you can't (yet) stick a quad core CPU with a high end graphics card into a small chassis with a single cooling source, the technology just isn't there yet -- the thing will run hot, and as a result the lightweight airplane will periodically attempt to achieve lift off ;-)
The Dell will of course run hot as well when under load, but the cooling system is superior, the net effect being a quieter work environment (compiling from source is a noisy affair), which is far more important to me than anything the MBP offers.