CALs

User CAL ? or, Device CAL ?

Scenario 1: Say, we have a small office with a Win 2008 Server and a fixed
number of PCs (say, 10). Is it best to buy Device CALs for this environment
? Meaning, it doesn't matter how many users come and go, there will be only
10 device licenses needed.

Scenario 2: Say, we have an Exchange 2010 server running on top of a Windows
2008 server. This server will be mainly for serving email messages;
meaning, all of the users will be connecting through RPC over HTTP proxy
component (and not locally) to access their emails. Even though the base
server is Windows 2008, there will be no user connecting for file access or
print access, etc. Again, only for email access. So, what would be the
CALs that I need to buy (assuming there will be 100 email accounts created)
? Device CALs ? or User CALs ? How many of each for Exchange 2010 ? And,
how many of each for Windows 2008 ?

Please comment.
Thanks
Steve