We have both and rarely use the convection in the microwave. You either have to be on shore power or ready to run the APU for all the time it is in use. As we are travelers and not camp ground campers we are seldom on shore power. But even when we are, using the microwave's convection unit makes the case very hot. Much more so than the microwave.
If you are seriously considering it, read the owners manual for the required clearance as it may be difficult to install in you galley. If ours should die and replacement is required, we will not spring for the convection again. It wasn't that much more as I recall, but it still just has not been worth it.
Matt