Hi,
Barrel availability is greater in 20 gauge. And probably cost of barrel may be cheaper.
A 16 ga. would beat out a 20 ga. comparing shot payload.
I just completed a Lehigh 20 ga. fowler. It's feel and balance are amazing, considering the barrel is 44"
Shooting a single round ball at whitetail deer would make little difference.
So the main consideration would be breech thickness, and barrel weight.
Good luck!
Fred