Whoever wins this will win with high difficulty no question.
While Sai is more of a long range fighter which has an edge over close range, I am slightly edging moreso towards Neji due to his byakugan. He will be able to see and counter everything that sai wouild throw at him since he uses mostly ninjutsu. Neji is also very quick being with team guy and has some ranged jutsu, such as air palm.
How would Neji win?
Sai uses his bird or the huge beasts he used against Deidara and won't let Neji breath.I think taijutsu here is useless since Sai is a long range fighter.
sai high difficulty. he makes an ink bird, taking away nejis advantage. then he makes Benevolent Kings, and ink explosive tags. neji loses his only advantage against anyone who can create solid clones. and he can launch a sneak attack using ink mist.