We sell a ton of fat bikes at my shop. For my money I'd go 26". The 27.5 bikes are all well and good, it's just too much tire for me. It's a bit like pedaling around on a tractor tire. The acceleration feels super sluggish to me because they weigh so much. That said, once you get them going they do roll out well. Also, there are only a few companies making rubber in 27.5 fat so keep that in mind as it may make finding good tires a little tougher down the road.
I have been running 27.5 X 3.8 Hotdags year round now. I will not be going back to 26 rims. I think the future is going to be 27.5 inspite of many 26 rims being sold now. Try a set they rip! For deep untracked snow you can go 26 X 4.8 or if you 27.5 X 4.5 My bike Pivot Les Fat can take both tire sizes. I do have the Mastadon pro ext front fork though. Enjoy your fat bike!