If I ordered the 20GA to save money, I'd always wonder why I didn't pony up the $120 for the 12GA.
You'll go deaf...and you better use a buffer, I've seen those stocks split on a 12ga. Maybe a bad batch of stocks-who knows...I've wanted to do a saiga shotgun conversion for a while now, but I wanted to do it with the bullpup AK stock.
If I could get either, I'd get the 12 gauge.