they make stocks, forarms, and barrels for 20 ga. you just need to make sure they are for the 20 ga. and with the 20 vs 12, it depends on preference. both will nock down a bg at 7-10 yards just fine. and some food for thought, a normal looking shotgun will look better on you over a tacticool looking shotgun.
Remington 870 12 Ga. Supermag
Springfield XDS .45acp