I've never been to Hartshorne but I have been told that is's sandy like Alliare. 6 Mile has different siol all together. It's mostly dirt with some sections that are dry loose shale type rock. The place is very fast if you want it to be and can be a real challange in certin sections. Not a lot of climbing but enough to make your ride a workout if you want it to be. All in all 6 mile is a very fun park and if you ride it at a fairly fast pace you will def get a great workout and feel accomplished at the end of the 14 mile loop.
I was thinking about posting up a group ride there for saturday morning. If u want you should come out and have a go at the place.