I asked this a while ago. The general consensus was that in order to suppress well, the bullet should be subsonic. The .45 ACP round is essentially subsonic to begin with. The 9mm would require subsonic ammo, which would decrease terminal performance. You can debate 9mm vs .45 ACP all day long in standard velocity ammo. I, personally, prefer 9mm. However, if I had the choice between standard pressure .45 ACP and subsonic 9mm, the .45 ACP wins, hands down. Ergo, if I was looking to suppress a pistol round, I'd suppress a .45 ACP.
I am, however, not an expert.
Mike