Just curious 25585, why you use the 10" scopes over the 12". I just jumped from a 10" f/6 to the 14" -- other factors such as Go2 and heavier base structure make this scope MUCH more to handle, but not too much more. I'm seriously considering selling the 10" as I'm not sure how much use it will get now...
But to speak to the OP, I believe this jump in size is more than worth it. I considered 15" to 18" but cost and size and my age always pointed me back to the 14".
Quite a difference in weight and size between a 10 & 12 F5. I bought my 10 as a first modern Dob, having had old heavy 1980s models before. Then I got nostalgic for the 12 I used to have, so bought an OTA which can use the original mount.
That 12 F5 was harder on my back carrying, but taller so better for standing. Eventually I got tge F6 10 inch, same height as my 12 F5, but easier to take around, and also collimate. However the 10 F5 is short enough to be car tranportable, so it has its own niche. All are solid tube.
There is a difference comparing both aperture sizes, though only 2 inches different. Most notable with nebulas and galaxies, the former using OIII filters especially benefits from using those extra 2 inches. With globular clusters I am uncertain, my F6 gives darker sky and better contrast, but the light splash in my 12 F5 is brighter. For star colours in globs not much difference, in open clusters depends on the colours.
Moon, planets and doubles show better with my 10 F6.
To the OP, buy or try an 18 first. You will find out if its too big and get an idea of whether its size suits. If you feel happier downsizing later, at least it will be with experience. I tried a 14 once, decided 12 OK, but now a quality 10 inch does fine and gets used most.