Like Derek said, the swing is the foundation of KB work.
In my opinion, the swing has more carry over than the snatch. I mean, truly, the KB snatch is just a KB snatch, it's not like you'll learn to snatch a barbell practicing the KB snatch. However, repeatedly drilling that hip extension with the KB swing could carry over into other movements that require an explosive hip extension. I'm not saying the swing will teach you to clean, jump, or snatch, but just simply that the hip extension of KB swings could create some muscle memory that might make learning or developing other movements easier and faster.
I know that's not his argument at all, but it was just my immediate thought when I saw the title of the thread.
As for the argument, from a conditioning perspective, swings are better. I've gotten out of breath doing swings. It's hard to get out of breath doing snatches. Doing snatches, my strength fails before my conditioning does, so I'm limited by the fact that I'm weak rather than being limited by needing to slow down and catch my breath.