I see Neji winning this one. Sai's attacks deal with ink based creations and he attacks from a distance. Neji's Byakugan his rotations and his gental fist keep him at a solid defense against Sai's drawings. Neji is faster then Sai and he even has mid ranged jutsu. Odds are Sai's going to stay in the air and attack with his drawings because he has no chance up close. Once his ink runs out his jutsu are done. I just see Neji getting this because once Sai runs out of ink he has no more options and if he gets within range of Neji's attacks he's done.