Both Amazon and Dropbox Business seem to provide 200 GB/user (although DropBox claims you can request more, whatever that means (free?)).

It is hard to say how they compare on functionality (e.g. versioning, auditing, integration (e.g. Kerberos, AD, etc)). The Amazon web-site is very light on details. It isn't even clear if Amazon has some kind of desktop sync application which is DropBox's bread and butter.

I'd need to wait for a LOT more information on Amazon's offering before jumping ship. Things like versioning (rollbacks, etc) are non-negotiable for many business users and while Amazon's offering might have that the web-site could do a better job letting us know as much.