Neji is far superior in intellect, taijutsu, and has a great KKG. Neji mid difficulty.
Actually they are both quite intelligent. Sai has a 3.5 in intelligence and Neji has a 3 IIRC.
OT: It would be a close fight but if Sai gets on his hawk, I don't see how Neji can beat him. Still at close range Neji won't even let Sai draw out his hawk and he'd take the fight with mid difficulty. At mid range, I see a win from either one but Sai would lose since he can't keep up with Neji in taijutsu or speed. Also it takes time for him to draw his beasts so I think Neji can stop him from doing that.
This battle would be very interesting at long range since Sai can send a large number of beasts to try and overwhelm Neji. Still Neji should be able to defend himself and destroy those beasts easily. I think once Sai goes airborne though, it is over for Neji since he can't get hit. I also wonder what types of other stuff Sai can draw other than animals.
Other things to note is that Sai is good at capturing enemies and stealth but this is useless against Neji because of Byakugan's near 360 vision and Neji's full body blow. Still if Sai has full knowledge on Neji, he may use Neji's blind spot to launch surprise attacks. Kidomaru was intelligent enough to pull it off and Sai is very observant too.
Here is list of animals that Sai can draw if anyone wants to know his abilities to their full extent.
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