We have migrated from AppMon to Dynatrace. OneAgent is really good as we no need to integrate with our services like AppMon agents. Just install OneAgent into the host and it identifies all the processes running inside that host. License works in different way. Since your question is very generic, please refer below link where you'll get to know how it works,
Let me know if you need anything specific related to Dynatrace OneAgent where I can help you with. Hope this helps!
This isn't a direct AppMon to Dynatrace Comparison, but this page comparing Dynatrace (What you refer to as OneAgent) to other 2nd generation monitoring tools does a good job of highlighting a lot of the capabilities you would gain from converting from AppMon to Dynatrace: