is there a major change in the content when choosing between sandbox and Visual novel ?
Or is it just the gameplay that changes ?
If by "content" you mean lewd scenes, I guess not.
But if you mean the game content ... I didn't play VN, but I'd say it's more than major.
There are activities and minigames that I can't immagine to fit into a VN. I don't know if duels are a thing in the VN, but to get to the last guy you need a LOT of practice (ok, grinding, but you can use earned money to buy enhancements).
Also, season 2 has a VERY long intro. The difference between that VN style story telling and the sandbox game mode later is like night an day.