the #9 torch (125amp A) will interchange parts with the #20 (225amp W)
the #17 torch (150amp A) will interchange parts with the #18 (350amp W)
also the #26 (200amp A) will interchange with the #17 & #18.
for long air cooled use the #26 is said be be much nicer than the #17. this should give you an idea as to what all will interchange. if you intend to do mostly water cooled work and only use air for a few quick small projects it might be a good idea to get a #9 for that and keep all parts interchangeable. you torches will be small and easy to handle. use the #20 for thick or long run jobs and the #9 for the little stuff you don't want to mess with dragging along the cooler. if you intend to do a lot of air cooled work away from the shop you might want to get a #26 and keep it in a to-go bag for on the road work. the #26 will stand up to the Dyn in the field much better. TIG is expensive no way around that. but torch parts don't wear out every day. investing in a 2nd size is not so bad an option.