In flight ballistics willusually bevirtually the same between a 20 and 12 with the same bullet being used out of the same length shell... IE)both 3" shells firing the Barnes tipped expander bullet.However the 12 gauge projectile will be considerably heavier for more kinetic energy down range.

There are some slug loads, such as Hornady, that the 12 gauge starts out about 200 fps faster than the 20 so the 12 will obviously have better in flight ballistics.

The 12 gauge has a distinctive edge in velocity, range of projectiles available, range of factory loads available, availability at stores, energy on target.

IMHO unless you are young and/or of small build I would get a 12 gauge.