Two very opposing sounds and looks. King Diamond is one of the 80s Metal icons while Junior Senior was a small indie pop rock band mostly known for one song. While King Diamond is all about bleak blackness and sharp whites with death being a huge portion of his music and lifestyle being a satanist. Junior Senior tends to have celebrated life in a bright colorful disco style.
Junior Senior's biggest hit was their 2002 track "Move Your Feet" which, in addition to having a fantastic pixel art video, is almost painfully catchy. Good luck getting it out of your head.
King Diamond has mostly had success in the netherlands areas with only slight success in the US on a few songs. King Diamond's falsetto is pretty iconic and has had some obvious influence on Mastadon.
King Diamond has worked with members of Motorhead, Metallica and appeared on the adult swim cartoon Metalocalypse as a fast food manager as well as the Blues Devil.
Meanwhile Junior Senior has worked with the B52s and were on the sound track for the much maligned "Looney Tunes: Back in Action" starring Brendan Fraser.
Jeppe "Senior" Laurson actually co-wrote and produced Lady Gaga's second most famous song, Born This Way.