Other than range, KE, and range of options the only real advantage a 12 gage has is that honkin' chunk of lead it throws, and it won't kill a deer better or deader. Most places where you find deer don't lend themselves well to long shots, the 12 is heavier and kicks harder, and the shells are often far more expensive too. My 20 is my go to gun for deer and pretty much any other game since I'm just a shell and choke choice away from anything I hunt. Unless you hunt over long open fields 90% of the time I'd go with the 20 gage.