The difference between 26 and 27.5" is all about the volume of air in the tyre. You need much more pressure in the lower volume 26 so they are much less absorbing of a rough surface. This means less grip, comfort and speed from a 26. OK it may only translate to a couple of seconds on a 1 minute downhill but there's also less chance of crashing. Over a well chosen cross country ride, though, my Whyte PRST4 can be faster and easier than my 27.5. Less drag and way less weight. It's horses for courses.