IMHO having a pre existing supply of ammo is the only good reason to get the 410.
I've had 410 Contender and a old O/U 410 and am well aware of the ballistics and capabilities, I have no desire for another one.
The 20 shockwave looks interesting I doubt a guy could tell the recoil between a 3/4 oz 20 and a 11/16 oz 410 and the 20 will pattern better.
Maybe not. 410 is cool gauge though and I like it. Patterning of birdshot at 10-15 feet is not an issue.