The Sager is well worth the extra hundred bucks or so just for the 870M vs the 850M in the MSI especially if you game much. Plus you'll probably get much better support if something fails than you would through MSI.
You know, to my experience and the general consensus I get through various sources it seems that for the most part Sager is a fairly well respected seller and has good customer service. I did notice the page your link took me to said they were back ordered though. MSI has well known quality control issues and I've never had anything but hassles when I've dealt with their customer service.