4.0's are a little more 'turn key" so to speak than the 2.5 because running bigger tires it helps having more torque. But ,if you love to tinker and do use it more for off road the 2.5 is pretty amazing. I look at my Jeep as a portable generator/stereo/tool chest,,,I have 2 optima hooked up to a 136amp alternator and I can run pretty much anything with an aux battery. Shit,,,,, I can make coffee, refrigerate my drinks, run a wench, have 10 lights on, and hook up anything that requires power. I can even run a solar panel to charge the aux battery. I can run all the stuff you can pretty much imagine.
I am obsessed with getting every little bit of power in my 2.5l and I love working on them, I have a rocket ship already with my new Acura but I get more head turns with the Jeepster. Having a convertible is pretty sick too. lol
My rule of thumb is condition....I will buy anything that has most of its life still left.