Over the years, the wheel size debate has been hotly contested. Trail bikes take advantage of both wheel sizes, and historically if you wanted an agile and maneuverable ride, 27.5 was your ticket, while if you were looking for rollover and speed, you'd want a 29er. However, as things have progressed, brands seem to be embracing the larger wheel size.
Because a 27.5-inch wheel with a 2.8-inch tire is roughly the same diameter as a 29er with 2.3-inch rubber, quite a lot of frames designed for bigger wheels will be compatible with both.
And if you want the best of both worlds, lots of bikes are available as a mixed-wheel mullet option, with a 29in upfront and a 27.5in wheel on the rear.