Hi Greg, I was forunate to practice with both before I purchased my CS.
If your HP-C has the .3-mm nozzle and not the .2-mm then there isn't too much difference between the two. That is what was in my friends at his body shop that I practiced with, the .3-mm nozzle. The CS had the .35-mm nozzle. The HP-C was a bit easier for finer detail work as it produced a smaller over-all spray pattern. The CS took some practice for creating the same fine details (close to hairline) but offered a larger spray pattern, which is what I need for my car modeling.
If you own one you certainly wouldn't need the other.
If you were trying to decide which to buy as your only AB, the below statements should help you pick the correct one.
Will be doing mostly detail work = HP-C
Will be doing general spraying and detail work = CS