I'm a relative novice but I have a 29" Scott Spark and a cheap motobecane 27.5" hardtail. From riding both of those, I decided to get a 27.5" Pivot Mach 6 which I just got a couple of days ago.
This is my take so far.
The 29" feels more stable and is smoother over small closely spaced bumps. Some trails here have lots of roots and it's nicer on that. The rocks at monte sano park, though, were a bit rough with the 100mm suspension. Plus the large tires went where they wanted to and it was generally harder to correct my riding mistakes compared the the 27.5".
The 27.5" makes me feel like I have a lot more control. I can maneuver it to where I want when I want and can correct bad turns easier. The 29" seemed more like I was just along for the ride.
For my size (5'8"), I think the 29" would be best for XC and the 27.5" for rougher stuff. Then again a long travel 29" might be perfect for everything.