During the past years 27,5” wheels have become the standard size for mountain bike while some mountain bikes still are offered with 29” wheels.
In the past it there was no choice. You could only opt for 26” wheels. Then the 29” wheels came along for reducing rolling resistance. But the bigger diameter also has the downside of prolonging the wheel base and hence influencing handling.
Furthermore, you should take your own size into consideration as 29” may be the choice of taller riders while shorter riders also may prefer wheels in a smaller size. See or read about the pros and cons in the video or article.