I'd go medium wide - that way you can add a half pad if needed
My CWD with a medium tree fits TONS of horses.
Wintecs are great with changable trees
I have a bates saddle. The one I have now and the newer models have the changeable gullet system. This system allows you to change the tree size. You have to buy a separate kit for it, but it comes with like 6 different trees (xs -xl)
I have a 2012 CWD 2gwith a medium tree and have ridden many horses with it. It fits most horses well enough to not cause any problems especially with different pads. The only problem I have experienced is wither clearance, one of my mares has absolutely massive withers and it's a bit close but with the right padding no problems have come of it and she is happy.