I've ridden both. With a tire that large, there are no pros or cons to either and I don't have a preference. There is so little difference in the tire size that it really doesn't matter as far as ride characteristics or float.
The fat bike industry was starting to get stale, so they invented 27.5 fat to make you think it's the next best thing so that you sell your current 26' fat bike and upgrade to the much betterer 27.5 fat bike. Some people fell for it. Others can read between the lines.
With that said...It appears there is a slight movement by the major players in the fat bike industry towards the 27.5 fat.