I recommend doing both... although Kapan is considered to be much easier content wise, it is still highly recommended.. and USMLE World although new, has very high ratings amongst previous examinees as well... I have done USMLE World at least twice, and the questions and answers are really great.. there are educational objectives, and other sorts of cool learning tools... I would recommend that if time permits, its really in youre best interest, as far as learning more material, and practicing questions, to do both of them....
good luck!