-Both combatants at PRE TIME SKIP levels
-Both Neji and Sasuke at Chunin Exams round two level (no curse seal for Sasuke)
-Fight takes place in Chunin Exam arena
-Fight to k.o or death
@The Stegman: I apologize if this is obvious and I'm missing it, but I don't know which you mean by 'round two level': Do you mean the Forest of Death round, or the round in the Arena? I'd assumed Forest of Death at first but the picture for Sasuke got me mixed up. Clarify?
@The Stegman: Ah, thanks. In that case, I'm going to give it to Sasuke by a small margin. The speed he obtained during the month between was specifically designed to replicate Rock Lee's style of movement and combat, which in turn was designed to defeat Neji. While Sasuke isn't as powerful or as fast as Lee by a long shot, he's still faster than the eye can see for brief periods of time. While Neji can use his Rotation to block Sasuke's attacks, Sasuke's smart, he has the Sharingan, and he has ninjutsu to compliment the taijutsu. With the Sharingan allowing him to see Neji's movements and presumably react even faster due to his training, thus attacking before Neji can pull off the Rotation, Sasuke should be able to land a few fatal strikes and dodge Neji's attacks.
@JonSmith: Neji could most likely already handle Rock Lee's speed even without the weights, which is what speed Sasuke learned. If you remember the Rock Lee vs Gaara fight the only time Neji was surprised by Rock Lee's speed was when he opened the Inner Gates to use Primary Lotus.
I'd still have to say Neji would win this fight. He was definitely the most powerful Genin in the tournament at the time. Possibly more powerful than Gaara? I don't know, that would be close.
@ohgodwhy: I doubt Neji would have been able to do much against Gaara, as the latter could simply create a bubble of sand and blast Neji until he won; Neji, whilst intelligent, fast and skilled, doesn't have any attack that's particularly powerful. Still, that's another fight.
This fight is actually really close I think, makes a change from all the Vegeta vs ultimate Gohan ones we're been having.
I think it would be extremely close, and could honestly go either way. Chidori would pretty much be useless here as Neji, with his byakugan, could see it coming a mile away, but then Neji would have trouble using his 64 strikes on Sasuke due to his sharingan. Sasuke does have an array of fire techniques as well, but I'm not sure how good they'd be when you remember Neji's rotation.
It would come down to which of them can be more resourceful and battle-smart, and in that respect, I would give Sasuke a slight edge. If the battle drags on however, I see Neji winning.