I am planning on purchasing a Nightforce 2.5-10x32.  I am buying a .5" extended riser, and will use either ARMS medium or low rings.  The medium rings would put the scope a little over 1.6" receiver to centerline, and the low will be a little over 1.4".  Which would be the better option?  I know the MK12 used medium ARMS rings with the swan sleeve, which makes it almost 1.6".  I just need to know which would be more ideal.

Unless determined to have an authentic clone, ARMS is pretty low on my list of mounts, whether rings or otherwise.   If you don't need true clone, consider something better, perhaps Seekins.

I'm going for an MK12ish style rifle.  I already bought some of the medium rings, but then thought low rings might be better.

It's a personal preference issue, I like my scope centerline at 1.4". Iron sights are nominal 1.53". 1.6" is too high for me.

I can actually use 1.250", but it's tight. Anything between 1.375" and 1.5" is perfect for me.