I have not had the opportunity to monitor MARCS either, but having monitored Indiana Safe-T, which appears to be similar to MARCS, and MPSCS in Michigan, you will slowly find more and more stuff migrating to it, so you will gradually hear less and less with an analog scanner. The other thing to consider it that you will learn quite a bit about an agency's setup right when they switch and are learning about the system. You may also have a better opportunity to hear seldom used talkgroups then as well.
...jim