Communication between a BlackBerry smartphone and the BES server is encrypted and secure. With a hosted solution the data is then on the hosting company's network. So it comes down to whether or not you trust the hosting company to protect that data adequately. All communications between the hosting server and other servers on the internet may or may not be encrypted. Google supports TLS encryption of email, but you don't say if the hosting service you're contemplating does.
For a complete description of BlackBerry security see this page
, but remember that for a hosted BES solution, the corporate servers belong to the hosting company.