I would have loved to see that fight. Even Ten Ten, who was Neji's biggest cheerleader by far was impressed with what she saw in Sasuke during the preliminaries of the chunin that even she wondered who would turn out to be stronger between the two...she who knew Neji best
Tho I'd go with Kakashi's assessemtn and the Sasuke at the time of the prelims wasn't on Neji's level...but I'm sure Kakashi trained and prepared Sasuke during that month of training prior to the finals of the chunin exams to be up there and competitive in case he met Neji as/if both advanced. Its not like Kakashi to go into anything unprepared; especially when working with someone who was a genius like himself....by then I think it would have been very close, tho I still may have favored Neji
But the Sasuke by the end of Part I after the fight with Naruto @ the VoTe when Sasuke not only pushed his cursed seal to lvl 2 but also fully mastered his sharingan eye bringing out its 3rd tomoe in both greatly increased his powers to unbelievable heights compared to before; making an even more impressive fight...too much of a leap for Neji to compete with. If Sasuke was able to handle Naruto with ease when Naruto unleashed his kyuubi chakra(no tails), which was the same level that was overwhelming Neji during the chunin exam fight shows by that point Sasuke base mode with 3 tomoe sharingan was greater than Neji who was barely keeping up with Naruto at the same lvl. It was only until the kyuubi started taking over Naruto and finally unleasded the 1 tail that Sasuke had to transform, by which Sasuke was on even playing field and won
So by Part II even in the begining before he awakened MS; Sasuke was lightyears ahead of where he was by the end of Part II; no way Neji kept up but it would have still been a really great battle between 2 geniuses of their respective dojutsu...a can't miss fight that would have been even better in Part II cause of the major improvements made by both fighters makes a better fight in general vs where they were in Part I