Provided Sai can keep his distance it'll be a close fight. I think Neji high difficulty. Sai was in Anbu and is a competent ninja, Neji is just a great shinobi. His mastery of the gentle fist and the byakugan put him at an advantage over at least 90% of shinobi.
Hmm IMO i would have to give it to Sai. Neji is damn good, but there is something to be said about ANBU level. Don't forget Sai was Danzo's prized pupil, tasked with infiltrating Orochimaru's lair and assassinating Sasuke - he is clearly very competent. After all in the war he landed the final blow on Deidara and Sasori and basically sealed the entire 7 swordsmen of the mist.
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