I say its a tie between them because they are both the best Taijutsu fighters in the Leaf(Along with Hiashi,Hinata,Lee,Kakashi). They have very different Taijutsu styles which leads me to think this would be a stalemate because Guy's Strong Fist style of Taijutsu is meant for breaking bones while Negi's Gentle Fist style of Taijutsu is meant for disrupting chakra and destroying vital organs.
Even if Guy opens 6-8 Gates I can see Neji just using Rotation and other delaying techs to wait for the effects of the Gates to wear off. It is risky to Guy to stay in the Gates mode just as it is risky for Sasuke to stay in Susanno for too long.
The longer Guy stays in the 8 Gates mode the more wear and tear it does to his body, so if he cannot finish Neji quickly he would most likely succumb to the negative effects of the 8 Gates and either not be able to fight or be very exausted. So if Neji can stall until it becomes unbearable for Guy to continue in that state he would definetly win. Even if Guy opens the Gates and then closes them knowing Neji's strategy after a while he will still be fatigued by doing so.