This has been a bone of contention in mountain biking circles since 29” and, more recently, 27.5” wheels were introduced and then largely superseded the previous 26” standard.
Old-school cynics said that the new wheel sizes were driven solely by the bike industry and that for their nimbler handling and faster spin up speed, 26” wheels were better.
In reality, 26” was only the standard as this wheel size happened to be what the original ‘clunker bikes’, which mountain bikes evolved from, happened to have and die-hards clung onto. There’s no doubt that a bigger wheel will roll faster and handle obstacles more easily. Plus there’s no real weight or spin-up speed cost to pay.
So, 29” or 27.5”? If you look at elite XC MTB racers, they’ll all be running 29” wheels. But don’t despair if your MTB has 27.5” wheels as, depending on your tyre choice, the overall circumference can be almost identical to a 29” wheel. The difference in speed between 29” and 27” is minimal, and getting your tread choice and pressure right for the trail conditions will have far more effect.