It is pretty hard to define when the "prime" of both these women was. What years would you earmark as their prime years exactly.
I would probably define Venus's prime during 2000 to 2002,where she successfully defended her US Open title(2000 and 2001),winning Stanford,San Diego and New Haven en route to the US Open and winning the Olympic singles gold medal,reaching the semis of the AO for the first time,defending her titles in 2000 successfully and winning Miami in 2001,winning 7 titles in 2002 and reaching the final of the US Open again,only losing to Serena in the final.
As for Kim,I'd think she was at her best between end of 2002 to 2003,and 2005. Won her 1st YEC in 2002,defending it in 2003,won Indian Wells and reached the USO final. In 2005,won IW and Miami in a row and the USO title.
I think the prime of Kim is pretty hard to define because in between years she's had injuries/inconsistent results that resulted in a year that could be defined as "either good or bad" and in fact from the way she's playing right now I don't even think her prime is past. But for Venus,she's still playing well now(winning Dubai,reaching the final of Miami,QF of AO) but as compared to her prime in 2000-2002 she isn't as good or dominating in tournaments as she used to be.