By definition the most attractive services are the personal free ones. Almost all cloud storage services provide a free account with limited features and then build on a paid service by offering advanced options and more storage space. These 'personal' accounts are usually quite sufficient too for a single user—a document can be uploaded and a URL link sent to a client or associate quickly and easily.
Free and personal accounts often have limitations to encourage purchase of higher-level plans (for example, Flipdrive limits file size to 25MB for the free service and 1GB for its lowest paid service), with the highest level plans placing no restrictions except total volume of the storage space.
The 'business' accounts can vary greatly but usually add features such as multi-user access to an account, access controls, and even logging—which is great if you want to keep track of who's uploading and downloading what. There are usually no restrictions except on the total volume of data that can be stored (and in the terabyte range—such as ADrive's 10TB top-tier plan). For those services that offer personal-key encryption, this is sometimes only provided on the business accounts.
The sheer range of services available can make it a minefield for a CIO to determine what is an acceptable service. It doesn't help that some 'personal' accounts can offer limited business features such as shared access for teams, while 'business' services can be as simple as multi-user access— scaling up to the most expensive plans that can include installing dedicated hardware within the company linked to offsite storage. Competition is fierce, and so as a result are the services and the confusing array of options.
Hence we've put together a table covering 30 cloud storage services, their essential features, and which ones offer private-key encryption. With respect to security, see the table embedded below: