If the computer is just going to be stationary, you can consider an All in One (AIO). If you sometimes want to carry it outside, you can consider getting an 18" laptop. Comparing the two you mentioned -- the GT80 and the Alienware 18 -- the MSI specs beat Alienware handsdown. MSI's GPU is a GTX 980M versus Alienware's GTX 880M, also the SSD storage (512GB vs 256GB) is double on the MSI over the Alienware. Another thing to consider is that as of January, Alienware will no longer produce 18" laptops, meaning that the July, 2014 model will be your only option if you go Alienware. MSI will continue to produce 18" models and is the only computer company to do so. Rather than the laptop though, I suggest getting an AIO because you can get a 27" screen and it will cost you less. The soundsystem is Creative Sound Blaster, which gives you sound quality that will satisfy most moviegoers. Adding the AIO's Full HD to the mix and you've got a respectable home system that doesn't take up more room than a TV and can function as everything you need.
Xiaomi Mi Mix 2 review