It takes a tech novice to compare Alienware and Apple machines. Alienware is heavily gaming oriented, providing superior GPU performance for a lower price range than other classes, and a cooling system better built for lengthy gaming sessions. As a result of pushing higher performance, battery life often suffers, but due to features like Optimus and Speedstep, is still very useable on battery. Weight is considered on a per-person basis. While many mid-range laptops are not really heavy, the cooling solutions often push weight somewhat higher than the average laptop.
The Macbook Pro on the other hand (15" Retina Model), is closer to the jack of all trades. For many users, the excellent balance of parts, weight, and battery life make this a workable laptop for most scenarios, including occasional heavy use and even light gaming. However, if you have a regular use for compute power (3d rendering, heavy gaming, etc), the rMBP is a rather poor choice due to it's pricing, lack of expansion options, and cooling system that may not be up to hours (or days) of hard use.
tl,dr
Macbook: Well balanced machine, light weight, long battery life, thin (less space requirements for transport, at expense of expansion and cooling)
Alienware: Greater compute power, heavier, modest battery life, robust cooling, thick (allows for expansion, which may or may not be necessary to you)