I'm relatively new to MTB'ing but my limited experience is as follows:
I have a 26er MTB I've owned since 1999 but only ridden off-road occasionally, so it is still in excellent condition. Anyway, it is waaay slower over my typical 50km trail route with circa 500-800m of climbing than my 29er. There is zero chance I'd race it but for the odd fun ride, it's ok. Lovely for learning skills on shorter rides.
I've ridden 27.5+ and found I preferred that on more technical Enduro trails and downhill. More stable, for me. However, my 29er feels faster for XC.
I have an eMTB for fun too, it is more an Enduro bike set up with 160mm travel and has a 29er front and 27.5+ rear - that is a great combo for that type of riding.
Overall, I prefer 29er's but then I am biased towards XC racing.