What does Serena have that King doesn't have, other than the extra slam? King has many other areas where she far surpasses Serena. Is this a slam winners list? Just curious.
Serena had tougher competition overall. I know people will mention the people King has played but if you analyze closer King won most of her slams while Court was either taking a pregnancy leave or just coming back from one, when Bueno was already past her prime, and before Evert began to dominate. Serena directly beat greats when they were at the top of the game, even if only secondary greats like Venus, Henin, and Hingis to win many of her slams.
I dont know all of King's stats as they arent as easily available for players back that far. Serena though was the dominant force of her time in a way King wasnt in hers. There is some periods King was the best but she wasnt even close to the best player of her own era overall, while Serena is by far in hers (and yes I know Serena didnt face a Court but as far as how each fared in their own time that is still a big difference, and who knows what either Court or King would do in a more modern era but in Court's case you obviously have to acknowledge her record). In fact I dont believe there is a single year King was the best where Court played and was near her best. I guess you could argue 1966 since Court played most of the year and King finished it #1 but she probably wouldnt have if Court had finished the year as she was ahead overall at that point even with King winning Wimbledon. And there is 1968 but this was probably the worst year of tennis of Court's career, reaching no slam finals other than the typically depleted Australian Open and losing in the quarters of both Wimbledon and the U.S Open (not to King). King's best years were in 1967 and 1972 when Court didnt even play (except the very end of 72 coming back from another pregnancy leave).
Serena's longevity already surpasses Court. King won all her slams from 1966 to 1975. In her return to tennis from 1977 onwards she didnt post a single win over Evert for instance the rest of her career. Serena has won her slams from 1999 to 2010, a longer time span already.
King is also fortunate in her era 3 of the 4 majors were played on grass which is by far her best surface, even if she didnt fully capatilize by skipping Australia often (not that she would have had much chance vs Court on here home turf anyway. Honestly it is hard to imagine her being nearly as strong as Serena on hard courts, as she wasnt considered as strong a player on hard courts as grass. And on clay she is weaker than Serena despite that it is Serena's worst surface too, King's French Open was a minor miracle in a year Court, Evert, and Richey all didnt play (although she did beat Evonne which was a good win). Only on grass or indoors might she be as strong or superior IMO.
Obviously King does have some obvious edges- consistency, more Wimbledon and U.S open titles at this point, better overall tournament performance. Still overall I went with Serena by a hair. The two are interchangeable at this point really.