The 9mm conversion barrels are fine for just tinkering or playing at the range but I wouldn't depend on it for any defensive needs. About the only conversion barrels I would trust for defense would be the 357 SIG/40 or 45ACP/40 corbon mix.
So I would go with the 17 so it will always work reliable.
I have a G31 with a 9mm conversion barrel, and it always works. However, it does not eject the same as a 17, seems like less power. It would not be first choice for a defensive 9mm handgun. But it does make an awesome plinker if you are just looking for cheap ammo to shoot. (if you can find any)