Besides looking at licensing costs, the fundamental issues is whether you wish to use an SaaS (software as a service cloud) solution and if not do you wish to invest heavily by running a SQL server. One key issue is synchronizing with Outlook and Google calendars. Many vendors offer multiple services. See chapter 14 of Sharon D. Nelson, John W., Simek & Michael C. Maschke, The 2016 Solo and Small Firm Legal Technology Guide (ABA Law Practice Division) for more detailed information than is given here.
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