Seems like the biggest debate in mountain bikes for trail riding (versus cross country and downhill) is the choice between 29 and 27.5 inch wheels. I have recently demo'd several 27.5 bikes and I own a 29er Stumpjumper so have been able to compare a bit. I find both are great and have their advantages. The 29er is known for rolling efficiency and straight line power. The 27.5 is considered more playful and fun. I have found a key factor on performance of either is tire width and tread. Either build with a narrow tire and poor tread does not ride well. Said differently, a nice tire width and tread can make a big difference. Many bikes come with a tire width of 2.3 and now plus sized tires come with 3.0 inch tires. I find the 2.3 is fast but a bit narrow and the 3.0 provides a great ride but a bit soft around turns. I have talked with many in the bike industry and the feeling is the tire widths for trail bikes will settle in the 2.4-2.6 inch width. I recently put a Maxxis DHF Minion 2.5 (photo below) on my front wheel and it is sweet. It is not fast uphill but provides great traction and confidence downhill. I run a 2.3 on my back tire for better rolling and lower weight. Mountain bike gear is ever evolving so it would be great to hear what others are experiencing.