The 500 will give you a lot more upgrade/customization options then the SA-20.
You are very limited with stock and barrel options with the SA-20. The 500C will use most od the same stocks as the 12ga (500A) model.
I have a model 500C and I have the a field (26 or 28"), turkey (22") and security (18") barrel for it. It can easily adapt to whatever I want to use it for.
If they only offered a deer barrel for the SA-20, I'd have one already. LOL