I have a client, a law firm, who needs to have 10GB of data uploaded to a secure FTP site or an online storage service so all of the lawyers can access the info, change the files and reupload them, etc. The service should allow multiple users to access the site at once in case more than one lawyer wants to access the site at once.

Which would be the best way to go, an online storage service or a FTP site (I know they are kind of one in the same, but not always)?

Any suggestions on which company would be the best for this and most secure?