"300 ultra mag vs. 9mm"
Well, let me lay it out straight for you, random Googlateer! Abandon all confusion now!
The 300 ultra mag is a very, very large rifle cartridge. The 9mm is a relatively small pistol cartridge. I believe the difference in muzzle energy is roughly off by a factor of 10 (about 400-500 vs 4000-5000 ft-lbf).
Here's a handy pictorial representation, from back when:
9mm third from the left, and 300 RUM the huge one on the right.
In laymans terms, that means if you get shot by the 9mm you will hurt. A lot. Probably won't want to do anything but fall over and clutch your wound. If you get shot by the ultra mag, you probably won't care, and the guy behind you probably won't care, and the guy behind him will probably fall over and clutch his wound.
Remember, as a rule of thumb, all pistol calibers suck. And if you are trying to compare it to something that is basically a stretched 30-06, which has ultra in it's name, well... there is no comparison.