The "natural" progression would be from MCSA on Windows Server into MCSE on Windows Server, since the MCSA provides a base for the MCSE.
The MCITP on Exchange server is a completely different path. It requires two exams on Configuring and Designing/Deploying Exchange Server, which are unrelated to the MCSA that you already took. This doesn't mean that it is not useful. If you are finding
a lot of requests for doing work on Exchange 2010, it would be a good thing to have the corresponding certification. But it doesnt "follow" from having obtained a MCSA on Windows Server.