This question is directed to people who own and/or have tested both of these Muramatsu flutes.
I'm very happy with my EX, and it is probably a higher quality flute than I need, and possibly will ever need.
Still... I'm human, and not immune to "flute lust". I like thinking about newer and shinier flutes that will revolutionize my sound, tranform my personality, and solve world hunger. I really hate to ask Muramatsu or some other vendor to send me some trial flutes just to satisfy my idle curiosity without much likelihood of a sale. So I'd like to get objective or subjective thoughts from anybody who has played the EX and the next model in their line, the GX.
Muramatsu's literature doesn't provide much to go on to compare these flutes - they simply note the higher silver content of the GX. Is it an exactly identical flute with more silver, or is it created with more precise standards of engineering? What does it feel like to play one versus the other? At what point would you say to someone, "you really should be on a GX, not an EX"?
I love Muratmatsus. I have an EX as my secondary flute and I would go as far as to say that it's the best flute for under $8000. I have tried the Gx and would say that your tone doesn't really depend on the silver content; it depends on how made that particular instrument and headjoint are. Playtest two Ex flutes side by side and you'll notice some difference; I picked my current EX after trying 2 other EXes and a GX. As far as the mechanism goes, I would say that perhaps the higher-range Muramatsu flutes are better built, and perhaps not. I don't know for sure as I've never had to repair either of mine. If you don't want to go to order flutes for a trial, go to a flute fair. You might find flutes from other companies that you like.
You should only go with a GX if you've tried a few EXes and felt that they aren't as good as the former. I actually thought that my EX was better than the GX I had tried, which leads me to believe that, when it comes to handmade flutes, one should try all possibilities.
I could never try any EXs or GXs before I bought blindly my current EX-III. I'm veeeeery happy with it. It's my backup flute.
I'd say to you to stay with the GX if you play as a professional in a daily basis and demands more durability from your instrument. It's the only difference I could point to you between these two models.