My first turkey was shot with a 20 gauge modified choke at 15 yards.
I bet with a full choke or turkey choke with that 20 gauge you could shoot 30+ yards
The 12 gauge would be fine for closer ranges (10-20 yards)
but i would go with the 20 gauge and a full or turkey choke.

you don't need to be able to shoot 50 yards to kill a turkey,just call them and be patient,the longest shot I've ever got was 30 yards, but i waited and it came in to my decoys (about 15 yards away).