Definitely the Asus. It wins in every single area with the exception of the legacy drive speed, which doesn't even matter considering you'll have twice the SSD space. I don't know much about Alienware's warranties, but if you exercise any degree of caution when it comes to your pc, (not spilling things on it, carrying it in a padded backpack, keeping it on solid surfaces when gaming ((airflow)), etc etc) You shouldn't have to worry about a warranty.
There's no question as to which has better specs, the ASUS is a better laptop, the only question is do you want to sacrifice 16 extra gigs of ram, and 256 gigs of SSD space to get the better warranty (if Alienware even has a better warranty than ASUS, I've never really looked into it).
It's really hard to say whether or not something will suffice for the next 5 years in gaming. PC gaming is such a broad subject, I get by on an 8 year old mid grade GPU, and I play next gen games with decent framerates. That being said, I play on low settings to get those framerates.
With these choices, I would say if the past is anything to judge by, you should be able to play games just fine on medium-high settings in 5 years. Here's a link to a video on youtube of someone playing GTA V on a 5 year old x80 series GPU, and they're getting great framerates on high settings.
https/www.youtube.com/watch?v=_NNNtgKdhMg
You'll have an only very slightly out-dated high end graphics card which for 1080p gaming should last you quite awhile, way more than enough ram, and a top of the line Intel Processor. Like I said though, pc gaming is unpredictable, and its hard to say exactly what direction pc gaming will go in the next 5 years.