Well if you favour mobility you need to consider size and weight, because believe me, the M17x and the M18x are MASSIVE and weight a TONNE! My cousin has an M18x and he can hardly carry it in his backpack for extended periods of time!
Performance price ratio. When buying anything from alienware, don't just add upgrades. Upgrade things you think you can't upgrade later, but you will always be able to upgrade Ram, SSD, HDD, and even the graphics cards and processor on the m17x and m18x for much cheaper on your own. Note: Ram and HDD/SSD are easiest to upgrade on your own, where graphics and some models that have socket CPUs require extensive knowledge and confidence to take the laptop apart.
Also consider what you want to use it for. If you like to game with great FPS but don't require that everything is maxed out on the latest and most demanding games, then the most expensive models (m17x, and m18x) are probably not for you. Instead go with the smaller cheaper models.