Its hard to know what laptop will last longer until they are out about a year or and they start coming in for repair, so people like me can start getting a feel what the common issues are. I'll keep my comments mostly on design as I don't pay much attention to cost of laptops and features they come with. Also I have very little experience with Dell as I think I have only worked on 4 or 5 dell laptops.
Almost all laptops have some known issues that come about through bad design or bad hardware, this is true of all brands. HP has in my opinion some of the best dc jacks that almost never go bad. Virtually all HP now use dc jack harnesses which is a dc jack on a wire that connects to the system board through the wire instead of being soldiered on the board. The plastic case usually holds the dc jack in place and absorbs any torque instead of the soldier joints which will crack over time.
The display hinges are a huge point in failure on a lot of laptops and HP typically has it figured out in this regard, but I can't comment on dell. I can comment on a lot of new gateway and acers like the NV5468u which I have suddenly seen a lot of broken lcd back cover and bezel around the power button area (bad design).
Led displays are a huge step forward in my opinion and I would not buy a new laptop unless it was a led lcd. To date I have not seen one led display back light issues. Just be careful with it though b/c it is typically thinner and pretty easy to crack.
As you probably know solid state hard drive > traditional hard drive in durability, but the lack of space on solid state really sucks atm.
Really not a lot more I can say with out you first taking the laptops apart. However I can say laptops that don't have easy access to the fan on the bottom (typical of hp) make them a bit harder to clean out if you don't have an air compressor.