Updating the downloadable version requires building a setup-package, tuning the database-upgrade tool, updating the installation manual, testing the installation package on test servers, publishing the package on our website... While with the hosted version we just roll-out updates instantly.
We try to deploy new version as often as possible – sometimes even multiple times a day. There are no service interruptions, so our users do not notice a thing when we roll out updates. That's why new handy features, bug-fixes, design-improvement and other cool stuff appears in the hosted (saas) version first.
However new Server version releases are pretty frequent too. On average we release a new server version once a month, unless there are critical bug fixes that need to be released immediately. Updating the server version if not complicated, but it requires some work. This is one of the reasons why we recommend our customers to choose the hosted version. If you don't want to have any headache with server maintenance, updates and weird Windows issues, go with the hosted version.