If it's 8 gates then Double KO...
Assuming Lee isn't suicidal, if he's able to go to 7 Gates, then it leaves Neji with a slim chance at victory to say the least. Gai stated that Lee's high-speed combo from the Ura Renge is the key to defeating Neji.
But considering they're rivals and that by now Neji's likely accepted Lee as his equal, he's probably been working on this vulnerability.
When it all comes down to it, asking who would win between Lee and Neji is like asking who would win between Gai and Kakashi. As rivals, they're both capable of beating the other