That's a good question. As you may have seen the thread about a fat bike being the only steed in the corral.
I personally no longer have an MTB, the fat bike replaced that, and other than some more effort on the uphills, I am more than satisfied with that.
As far as which wheels? You may consider just getting new tires, (3.8") if cost is an issue. A wheel set will cost $400-1,000, plus the tires, cassette, rim tape, tubes, discs. Double that if you want to go carbon.
Any way, in the end, I think a 27.5" X 3" would be a good match to your bike. Just not sure how common a 27.5" rim comes with fat bike hub widths
The 27.5"X 3.8", I don't believe would give you any weight savings.
Let us know what you do. Cheers