Depending on what rules you try to define, you might run into a limitation in Dynatrace where it restricts what classes can be defined as a custom entry point. AppMon did not have such a limitation. This likely could happen when defining a rule such as java.lang.x.y.z. These types of rules can be defined but requires manual editing of the associated JSON text, something only possible with elevated privileges thru the debug UI. There has been discussion about permanently putting in rules for rich clients, but in the meantime this workaround worked great for me for a recent opportunity which sealed the deal. Ping me for details.
We are currently evaluating for a customer where both appmon and SaaS/Managed are in use already, so both would be an option for that application.
For SaaS/Managed, we would instrument the application with a PaaS agent, then define the custom entry points?