Realistically, they're not comparable. The Alienware 17 is much larger and more powerful. The Razer Blade is 4.5lbs, the Alienware 17 starts at 9.1lbs and goes up from there. Realistically a ~10lb laptop is not really a laptop but more of a portable all-in-one workstation.
If you want a small laptop that's still fairly powerful, get the Razer Blade (there is nothing in it's size class that comes close performance wise). If you want one of the most powerful gaming laptops around and don't mind that it's huge then get the Alienware 17 (but make sure you get one with a Geforce 880M because the 860M is too slow for such a large laptop). Build quality on both is very good, I know people who own both of them.
Another laptop to consider would be the Alienware 13, which is a smaller lighter Alienware system with a 860M. It's priced a lot lower than the blade and supports an external GPU (optional). It does, unfortunately also have a low-voltage CPU.