Revit MEP was designed for the BIM workflow. BIM is about building a model and being able to leverage that model. You might leverage it to do better coordination between trades, perform analysis on the model like being able to do an energy analysis, or simply to have always up to date sections and annotations on sheets.
AutoCAD MEP is capable of all the above, but was not originally designed with a BIM workflow in mind. In many instances, you have to manually regenerate views to keep them up to date. You can make a 3D model, and analyze that model. However, you would export that model to a 3rd party application to do an energy analysis, like Trane Trace.
AutoCAD MEP is an AutoCAD based workflow, and Revit MEP is a BIM/Revit based workflow. There are many users that use AutoCAD MEP for "BIM" on their projects. You will have a much better experience with Revit MEP for "BIM" workflows.
In summary, both can do BIM but Revit MEP was intentionally designed for BIM, has more native analysis capabilities and will give a better experience. AutoCAD MEP is a very mature product and fits nicely in an AutoCAD based workflow where the end goal is not necessarily BIM, but to get detailed and accurate MEP sheets out the door. It's very flexible as it is built ontop of Core AutoCAD and has all the flexibility that provides. In the end, they are different tools like a hammer and a screwdriver. Give both a try and find out what works best for you.
Hope this helps,
Joshua
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Joshua Benoist, PE
Senior Premium Services Specialist
Global Services
Autodesk, Inc.