The 9mm version will be quieter on a 9mm host. A 9mm can is also going to be smaller, lighter, more compact, etc. I'm a proponent of buying a dedicated can for each caliber. Suppressors are quite addictive and most people will just end up buying cans for each caliber down the road.
I see a lot of buy the "one can fits all" threads. While I don't disagree with that at all, I find that I get the most value out of my suppressor and tax stamp for that matter, when I have a can specifically designed for a particular caliber.
I see a lot of people shooting .22 thru their 9/45 cans or .223 thru their .30 cal can. Although it is done frequently, I think that the best performance is achieved when you use a can for it's intended caliber.