I'am not good at describing sound but i will try,The 6H8C sounds more relaxing than the 6DN7 a little less Bass and smoother treble.
To my ears there are times that the 6DN7 can be harsh on my ears but so far i did not experienced these with the 6H8C.
IMO when it comes to music/audio gear it's most likely a personal taste,Sounds that is good to my ears might not for others it's like one of those cable/capacitor debates but lets not go there.