The real cool bikes are 650b now FWIW.
The larger the wheel generally the larger the tire's contact patch and thus greater traction. The larger wheel will weigh more but will have more momentum once up to speed. The larger wheel will roll over trail features a little more easily. There is more choice among 26ers, but 29ers have been very popular now for a few years, so there's quite a bit of choice there as well. 650b is fairly unknown but starting to get a little traction (pun intended).