I would use the .22, a make shift noise reduction device especially with a sub-sonic round is to tape a 2-liter pop bottle to the end of the gun, hard to aim but you would be surprised. Also hollow out a large baking potato, about 1/2" hole place over the end of the barrel, also hard to aim but it works.
Skip the poison if you have any critters around, or even Hawks, Eagles, or Owls that may then eat the poisoned opposum.
I have the easy way, our Great Dane/Lab hates opposums, and she is good at killing them. She even killed a full grown racoon this summer, not a mark on her. Our Hound (Bloodhound we think/lab mix), can sniff out just about anything.
steve