I have used both quite a bit. Each has it's strengths and weaknesses which pretty much track with the programs in general.
When Revit works it works very well and can be very fast for modeling mechanical systems, but when it doesn't do what you want it can be VERY hard to get it right. This is particularly true with gravity pipes (drains).
ArchiCAD is not as automatic in the modeling and adjusting, but you can tweak and adjust things with a precision that is next to impossible in Revit.
Each can be very good and very frustrating but in quite different ways. It would be great to have the best of both in a single product. Overall I prefer ArchiCAD MEP but it's a close call so I use whatever is best for the particular job.
Two final things. If you need loads, flows and such use Revit. ArchiCAD lacks these tools. It is really just a layout and modeling tool and not really for engineering. On the other hand if you are modeling for export to Navis ArchiCAD is much better due to the layering, selection and export options.
