I wouldn't get too hung up on the latest & greatest, when it comes to fatbikes. 27.5 wheels probably have more grip, compared to my 26s. Good tires are the biggest asset. I have lousy tires & struggle on steeper climbs & cornering. My buddy has a set of Surly Bud & Lou tires that run circles around my Vee Rubber Bulldozers. Fatbiking on snow IS NOT mountainbiking! Just a way of getting on two wheels & getting exercise in the winter, for me. 1-1/2 hrs on a fatbike is like 3hrs on your mtb.
Just read your post and could not agree more that it is NOT mountainbiking, thank you for that comment as it is exactly how I feel and get flack for it.
I just want to add that the torque on the freehub on fat bikes wears them out quickly so if you do go used then look into getting a back up for it.