That's a tough question for me to answer. Each was so different than the other. Kes was very appealing to me, with her pixie haircut, sensual voice, and intelligence. Seven of Nine was the opposite - in your face lil Abner body in a catsuit to appeal to young (and old) fanboys. Kes was under-used; Seven over-used. They could have easily existed together on the show, since their characters and stories had little in common.
Seven will always be associated with why Voyager went wrong, but I thought some of her eps were okay. The problem was that the writing became cliched (the writers were, at that time, soap opera television writers, and it showed). Neither character would have been served well in the later years of Voyager, so it's good that Kes left.