Kes was a better Star Trek character. She actually personified the spirit of Star Trek with her will to learn and explore. Her mental abilities and her personality made her unique. She wasn't the typical "action-character" like Janeway, Torres and Seven, more a contrast and a complement to them, something that made her very interesting as a character.
Kes was beautiful, nice, helpful and friendly but also strong-willed, determined, brave and curious. She had a way to get her will without using force or threats. This can be seen in several episodes, like "Resolutions" where she persuaded Tuvok to go back after Janeway and Chakotay, something Kim got almost court-martialed for when he tried.
Kes's only flaw was the ridiculous lifespan. However, there were hints in some episodes that her lifespan could be prolonged which I think would have happened if she had remained in the show.
Seven was the kind of character that we have seen in so many movies and series, the typical good-looking and sexy female hero, however she had an interesting background story and her struggle to overcome the years she was a Borg and trying to be human again was remarcable.
I agree that Seven was over-used and Kes under-used. But I also think that there would have been room for both of them in the series, it would actually have been interesting to see the inter-action between them.
As for the actresses, both did a terrific job and really gave life to the characters. I must give Jennifer Lien special credit here, considering the fact that she was more inexperienced than Ryan but still did a terrific job. Both actresses deserves best possible credits for their efforts.