There is a thread with a lot of opinions on this here:
https://unicyclist.com/t/the-future-of-26/
To sum up mine: the difference between 27.5" and 26" exists, but is not that big (it’s actually 1" different, opposed to what the name suggests). Both the gap from a 24" to a 26" and a 27.5" to a 29" are bigger, and feel a lot more significant for me. If you look at unicycles standing around you can also see this, I don’t think I’d ever have an issue telling a 24" and a 26" apart from a distance, but with 26"and 27.5", that is harder to do.
I would recommend buying 27.5" new, because that is where the bike market has decided to go, so there are a lot more tire choices there. In my opinion, having the right tire for the job makes more of a difference than 25mm of wheel diameter.
If you find a good deal on a used 26", I’d still consider it. It’s not like tires have completely disappeared for them, just less and less choice.
Final point: a 29" can do very well even on technical trails if you set it up for it.