I think comfort level for the rider is a personal decision. I will say that all of the County trees run wide; so a medium really fits like a MW or W.
I have a medium Stabilizer that's too wide for any horse I've put it on; I'm going to be selling it soon for that reason.
Now, I also have a Symmetry that I love and it's a narrow. Fits everything from a chunky large pony to a large shark finned wither TB mare. It probably wouldn't fit a rounder warmblood type. I love this saddle, it's more of a forward flap and more like an "eventing" saddle IMO.
I would call your county rep and ask them for advice. They should be able to tell you what tree size of what saddle would fit similarly. I have a Conquest and LOVE it. It's a M, and with a Mattes pad, fits my MN TB well. To me the tree is not overly wide, but I know one of the jump saddles does run wide, thinking maybe the Stabilizer. Good luck!