I started with a 26. But now after shifting to 29, I prefer 29. The primary reason is stability in turns and downhills, not to mention that it rolls over a lot more stuff then my 26 ever did.
When I tried to go back and ride the 27.5, it felt small. Although it was great for doing bunny hops, jumps, and berms, it does not have the endurance, power, and strength of the 29. My 29 feels like a rugged adult bike, while the 27.5 feels like a toy.
For you, it might come down to a personal thing. I could see where a 27.5 would be good in dual slalom or 4-cross racing. But all my events are XC, XM, or Adventure Racing. So the 29 feels good during those events, while the 27.5 made me feel like I worked twice as hard (even though I think it was mostly in my head).
So I would make mine 29.
Your choice may vary depending on what you like to race or ride the most.