510 is the bike both C&J and HL tried to emulate
I hope that was a pisstake and not a twisted fact!
They weren't building 510's in 1980 let alone 1976
The XR will be the better motor as it is a later design, 4 valves etc.
The HL will fit into the pre78 class where the C&J will be the better Evo bike (although the lack of lower engine cradle bothers me)
If your not interested in classes then just go with your heart