older generations of stuff usually lasts longer. (very hard to die haha!) to me, i get stuff for self satisfaction, for self rewarding.
i hope alienware is otherwise. they can create new slim models; but i hope they keep the bulky design for 17; maintainability and upgradeablity are important for long term use, so is the proper cooling for important components like cpu/gpu.
between the two, i will always choose alienware. if you're looking for 3 years or more of usage, like raymondtong94 said, get 17.

Sure

Cpu, gpu, ram, ssd (sata3 and msata; up to 4 drives), wifi card is also changeable.