Our house gun IS a 9mm (Ruger P95) because my wife's arthritis won't let her shoot a more powerful handgun enough to practice. A high-cap full of 124 grain +P Speer Gold Dots contains enough energy to take care of your 300 pounder. Don't matter how big he is, you put enough 9mm holes in him and he's gone. Since it's just one guy I would Mozambique him. If he didn't go down and stay down I still have 13 in the gun, another 15 in the spare mag, and I've given Thelma time to go for the Mini-14 and its 20 rounds of 55 grain Silvertips.
Dave's Rule #5: If you have to shoot at all, shoot until none of the bad guys are moving. Corollary: keep plenty of hi-caps handy.
That addresses your scenario. In general, I think the 9mm has gotten a bad rap because of the performance of a few subsonic loads that didn't expand, and that one shot in the infamous Miami Shootout that supposed should have killed the bad guy but didn't. IMHO modern 9mm ammunition is capable of doing the job. It may not hit as hard as a .357 or .40 or .10mm, but it's enough.