Having been a big fan of both, I'd definitely say Tales has had a bigger impact on me. I played KH first, but it was Tales of Legendia that got me into action RPGs and made me a huge fan of the genre. I have played almost every tales game after that and I am looking forward to new releases. KH...not so much. It's a good game but it just doesn't have the same appeal.
And I honestly feel most people would feel the same...except the fact that most of the market hasn't played them. I guarantee I can stand at my local gamestop and ask people if they have ever played any Tales game and most would say no. Yet, I know hardcore shooter fans, one I currently am roommates with, who absolutely LOVE Kingdom Hearts. They've played it, loved it, and are looking forward to further releases.
Why? I believe it's because of the inclusion of Disney characters. It appealed to their childhood AND looked like a RPG that wasn't "boring turn based game-play" in my friends words. Kingdom Hearts just had a bigger initial appeal to a broader audience. Tales games feel too Japanese without an American appeal for it to get mainstream gamers to give it a chance.