I too was faced with this dilemma recently. I've ridden and thoroughly enjoyed mountain bikes with 26" wheels for years because that was the industry standard. But when changing bike last year, I suddenly had two more options. I went 27.5" and they seem pretty good. Is there a massive difference? Not really TBH.
Unfortunately, it seems bike manufacturers are phasing out 26" wheels now, so what was the standard for years is suddenly seen as old hat. I don't subscribe to this theory, I merely see them being pushed aside for something different or "new". This is also an excellent opportunity to sell us all new bikes with different wheels sizes.
Marked improvement or marketing ploy, you decide....