2DF is waaay more streamlined, has a ranking system for a lot of games, has detailed stats from most games you play, has a recap system that lets you view EVERYTHING you’ve played on 2DF (shit, you can watch everything someone else has done too, great for spying on the competition ), and has tons of game support but is lacking players (especially ones from the US).
GGPO probably has the better netcode and does have way more people in Super Turbo. GGPO has had nagging glitches that have yet to be fixed for months though (like getting challenged when you are in-game, challenges not being canceled when you enter another game, etc).
And if you want challenge in ST on 2DF, just find Damdai. He’ll kick your head in as long as you want