I mountain biked a ton in the 90s then not again until last year. I got a 29er and it was an absolute revelation compared to what I was used to.
The larger wheel do occasionally have problems with low speed obstacles I can't get around but can't quite roll over. I occasionally have some trouble whipping the bike around super tight corners too. I have found it to be better 95 percent of the time and only occasionally would a 26" hold a debatable advantage.
I wonder if the 27.5 just splits the difference?
How about a bike that can handle either and just get two wheelsets?