smlekid:
I believe you will find the 28 every bit as effective as the 20 for most any purpose. Cartridges are much more expensive however. Especially if you use a 28 at sporting. If you have a copy of W W Greener's "The Gun And It's Development," take a moment to see what old William Wellington had to say about the 28. No greater expert existed.

Best, Kensal