If you are going to buy just one tire to go with your Hemisphere, I would get another Hemisphere. The matched pair will look better than mismatched sine the width differences between the two are really noticeable. If you do go mixed, then put the Nimbus up front. If you buy a set, then get the Nimbus.

I've used both tires and either one is a good choice. I've even ridden the Nimbus on some dirt roads without trouble, but was not very confident with them off road.