This is a really popular thread topic
Anyways, I’ll chime in cuz I have both sizes. I have owned two Nimbus 26", still have one, and I have both the old and new Nimbus 24" Muni.
For starts, I have been riding muni for one year, I typically ride rocky, rooty single track on varied terrain typical of Appalchia (Tennessee). My style is more XC in that I roll or roll hop over obstacles vs doing trials styles hopping and jumping. My biggest drops are 1-2’. I am 44yo this summer, weigh 200# and would consider my fitness level as high.
I was bouncing back and forth between the two sizes, but have settled on the 26". Both of my munis are set up with 3" tyres, the 24" uses a Intense DH, the 26" uses a Duro Leopard.
I don’t dislike the 24", I rode it 3-4x/week for six months, with a few rides on a KH 29er (me no like for muni), but I decided that a 26" muni might work better for my stye. I built a Nimbus 26" and outfitted it with a 2.5" TRY ALL. I liked the 2.5, but it didn’t have the volume I needed to cushion the ride, so I upgraded to a 3".
I have been riding my 26 x 3 for ~ a month, have not touched my 24" since, having no problems on the same terrain, really have no interest in riding the 24", though it’s still in my stable When I did take the 24" for a long ride after being on a 26", I felt that the 24" was much twitchier, even after I felt comfortable riding it. I think the 26" is faster and holds a line better than the 24", while still being a capable muni machine.
If I was 20yo and into a more trials styled riding, like my 13yo son, I might ride the 24" exclusively. This isn’t to say that the 26" can’t be used for trials or extreme muni, but the 24" is more agile and easier to handle on terrain where you need to do a lot of maneuvering.
Your size also has a lot to do with which uni is more comfortable. My 13yo is 115#/5’ 8", so to him the 26" muni is more like how a 29er feels to me. If you look at Kris, he can ride anything, but most often rides a 24", but then he’s much smaller than me.
You could get a 26", then get a 24 “second wheel”, so you could enjoy both, but the brakes would only work on the 26".
Also consider crank length and tire choice. I run 165’s on my 26" and 160’s on my 24", though I did run 150’s previously and did fine. A bigger tyre and steeper terrain/more aggressive style suggest a longer crank; brakes are a personally choice.
Another reason to get a 26": Tyre choices are much better, and this will likely be more the case over time since 24’s are becoming less common.