For such an itinerary, I would rule out a mini: OK for town and proximity driving but I think you will appreciate having at least an eco for longer distance.
You also need to factor in that diesel models tend to have a higher mpg than their petrol counterparts. If the difference is only $30 between a petrol eco and a diesel eco, i would have thought that you would easily make up the difference on that kind of an itinerary (assuming that $30 is the difference for the whole of the rental).