Seems to me S3 is more designed for highly reliable data storage. If you wanna store a bunch of data in a way that's highly reliable and resilient to hardware failures and also not have to worry about managing RAIDs and clusters of servers for the data size, S3 is just the thing, and probably priced pretty fairly.
It's also a capable and flexible service. It's possible to use it for things it isn't really designed for and have it behave pretty well. Well enough that you can mostly ignore that the other thing isn't really what it's designed around. The "mostly" is important, though. If you start hitting any extremes while using it in a different way, then you certainly can run into some pathological cases in the pricing structure and pay way too much for something that would have been cheaper in a service dedicated to that.