It depends on what you want. Both are high quality shotguns. The SX3 may be easier to get a great pattern, but with a little trial and error the Affinity will shoot right with it. The SX3 will be a softer shooter, but it will also be heavier.
I were to shoot a dot I would go with the Marlin 336 base with a Vortex Venom or Fastfire 3. That is as low profile/lightweight as you can go. Both have awesome warranties. I think you may have to get a gun tapped for the 336 base, but I think it would be worth it.
The INdian Creek .555 would be a good starting point for the Affinity with any ammo.
For the SX3 I would contact Sumtoy, or go with some choke in the .580ish range.