Between the tx2500 and the Latitude XT, I would probably go for the tx2500.
A friend of mine has it for university, and he gets a full day of computer studies and IT courses from the batteries you get with it (on power saver with wifi enabled). On power saver, you still get 1.4Ghz dual core processing, and on normal power, you get 2.1Ghz Dual Core (Turion X2 Ultra ZM-80).
It retails at £700 here in the UK, so a similar configuration is probably about $1500~ USD for a new machine. I think HP have an outlet, but I haven't looked. Maybe a Google search or a quick call would find that out?
I think the Dell is slightly sleeker, but I have not looked into weight, only knowing that the tx2500 series comes in at 1.95kg (with battery). In the US, it'd be the tx2500z though, so you might find it as that.
Hope this information helps.