Wlad is the technically superior puncher with more power in all of his shots. Because he puts his weight behind them. Wlad also has better balance, and is a little quicker. He's definitely more athletic than his older brother.
Vitali on the other hand, in my opinion at least, is the better fighter. While not as athletic, he has the intangibles his younger brother doesn't have. Vitali has better defense and a little better footwork. He's also adept in keeping the right distance and using his height to his advantage (they're probably equal in this regard). Obviously, Vitali has the better chin and can take the better punch. He's also ALWAYS more composed than Wlad, and it's hard to rattle him. Elder brother arm punches, but he does that because it suits his style, and allows him to escape his opponent's shots. That's why he very rarely commits to his punches. Vitali can lean back and slip shots better than Wladimir as well.
While more awkward, Vitali is the better all-around fighter. I think both Klits beat Haye, but Wlad takes care of business earlier than Vitali due to his speed and jab. On the other hand, Haye most likely has a better chance of beating Wlad because he could always land a huge punch that'll rattle Wlad, whereas the same hope is small against Vitali.....yet against Vitali, Haye has a chance of winning because of his speed. He'll be able to use his footwork and speed better against Vitali, but wont be able to hurt him, and eventually Vitali will start to get to Haye. Both brothers have more experience that will come into play. So, in other words, Haye is screwed. He's fucked either way, and I think he knows it.