I have run a 20" wheel most of the time I had my bike, but recently I had so much chain trouble, I returned to the 26" wheel. I have sense learned what the problem was but the idea of moving that sprocket again stinks. I might just buy a second sprocket and shove it on the 20" wheel.
But having gone from a 26 to a 20 and back this is what I have learned. The 26 is much faster. When I coasted down a hill with the clutch in on the 20" it wanted to jerk when I released the clutch unless I had the engine whining. Not so on the 26 it re-engages smoothly at about any throttle setting. The bike will also engage with less speed on the pedals but I have to be careful not to let out the clutch too fast or it will die. The twenty chokes out much less. Probably need to get used to it.
The bike is less stable with the 26" wheel. Also if I hit a bad pothole it rattles my teeth not so with the 20".
I think the twenty climbs hills a little better but both do it without stalling out So I can't really say one is any better than the other. I still have trouble mounting the 26" but I did drop the front wheel down to a 24" so I can keep my feet on the ground at stop signs. I think if I were to make a change for any reason I would go back to the 20 or at least a 24 simple for the ease of getting on and off the bike.
Anyway it's an interesting bit of worthless info.
My posts have entertainment value only. A bike ain't yours till it has your blood on it. Then it owns you.