Plenty of threads on this, I’m sure, if you search.
Keeping it fairly simple: given a similar tyre, the 26 is around 8% bigger, which means around 8% faster on the flat. Muni speeds around 6-8 mph means an extea mile an hour at most.
However, the bigger wheel is less vulnerable to small irregularities in the ground, and rolls over things more smoothly. Therefore, average and “journey” speeds are much higher.
On the other hand, the 24 is lighter, and more manoeuvreable.
The 8% will matter on hills. It’s like changing a gear on a bicycle.
Various schools of thought: a 24 for hard hoppy droppy MUni, a 29 for fast cross country. Where does a 26 fit in? Somewhere between?! Choice of wheel size is dependent on what is available. Maybe the ideal size is 25 inches, or 27, but you have to choose between 24, 26 or 29 because those are available. Is 26 the ideal compromise, or is it neither fish nor fowl?
My MUni is a 26. If I replaced it, it would be with a 24 with a fatter tyre, I think. The fat tyre makes the effective diameter nearer to 26. But then a fatter tyre on a 26 would make it a bit nearer to a 28!
More tyre choices available for 26 than the other two sizes.
Buy one of each!