I play on NES graphics because I get screen crunch when I play on modern graphics (might because I used winexp to make game window smaller so I can see livesplit and chat) and I notice other runners use modern and the leaderboards are split between the two. What time difference is there and which is faster?

the board isnt split for graphics, but rather separating the actual NES version of the game. The actual NES version likely has more lag, so my guess is that modern is faster. However, that lag could result in skips possible on NES but not on the other consoles/pc, so we will see.

While the 8-Bit mode version is an actual NES game, it is running on an overclocked emulator which is why it cannot be part of the same category as regular NES.