I have a can on a 9mm, 45, 308 and 5.56.
For me, I like the 9mm with subsonics the best. With 147gr subs, you can get good speeds with a heavy bullet and good sound reduction. Also, I really like the fact I can shoot a pistol caliber rifle (AR9) with the can and it's just that more accurate and fun.
The upside with the 45 is that its 230 gr are naturally subsonic, so any off-the-shelf ammo (for the most part) will work with a can.
My advice would be to get an Advanced Armament Corp Ti-Rant 45 with a 9mm piston. That way you can shoot it out of a 45 or a 9mm. Your sound reduction out of the 9mm won't be as good as a dedicated can, but the difference isn't all the at great.
You could even shoot 22LR with the adapter if you so wish.