To say that Ruutu will be the better overall player is more than a stretch. Frolov is already the best defensive forward on the Kings, is already solid in the corners and along the boards, and should he continue to put on the muscle, he'll be nearly unstoppable from the faceoff circles on in. He does need to use his teammates more, though revoloving line combinations and inexperience may be at the root of that issue. I see him topping out as a 35-goal, 75 point player with absolutely no flaws. He's sort of like crossing Jere Lehtinen with Ulf Dahlen, only with better hands.
Ruutu will definetly be more explosive, and is already more entertaining than Frolov will ever be. Sure he needs work in his own zone, but how many rookies (except of course for Frolov's first year) don't need that training? Ruutu should put up a few more points, but not enough to establish a clear-cut advantage over Frolov's complete game.
As a life-long Kings' fan, I'll admit that Ruutu is one of the very few young players/prospects that I would consider worth moving Frolov in a trade. Right now, I'm happy watching Frolov blossom.