I haven't done either, but would choose MVMCP since it is seasonal, and the HDDR is around all the time.
We are going to the HDDR next week for the first time, so after that I will have a better opinion.
I would think young children at WDW for the first time would want to see characters. There are no characters at the HDDR.
We are going for about the 6th time, so going to the HDDR is a nice change for us.
DJ