Most who get a second wheelset in 27.5 go wider. Plus size has some advantages off road, and you won't lose as much bottom bracket height. If you need the width, you won't notice the added weight. If you don't need the wider tires they will feel slow to accelerate. Frame geometry generally trumps wheel sized for handling feel. My Pugsley with a 26x4 front and 27.5x 2.5 rear is quicker steering than my Monocog 29er, as counterintuitive as it sounds. My Fargo has the widest tallest tires I could get on it (29x3 and 29x2.5) just to keep from hanging pedals everywhere because of the low bottom bracket.
I choose the width of tire I want to ride for the trail, I have all three wheel sizes. The diameter usually doesn't make as much difference as some think.