I've been shooting a 20 for about 10-11 years now and wouldn't try to convince someone one way or the other.
The reason I switched was a shoulder surgery so I wanted lighter recoil. After that, with TSS (we've handloaded for a long time) there was no reason to go back to the 12... 20 gauge was Lighter, shorter (I shoot a youth 870 with a 21" barrel), less recoil, cheaper shells, and the patterns with TSS (or other HTL loads) are more than adequate to kill birds. I just didnt have a good reason to take the 12 gauge anymore.