Like desmosome said, it heavily depends on what you want to do.
Of course sandbox game is less linear so its harder to create because you need a good tracking system for your game state.
So maybe VN would be the best choice for now.
But still, you could make something decent if you can create a well designed system and make your game systemic, which means that you have various systems in your game that interact with each other and could make your game interesting with much less content than needed for a VN.
It is also easier to expand and add content to it (again, in case it is designed right), every new content you add could've much more impact on your game than just adding some new content to a VN.
Also, in a sandbox game you could get away with shallower story and characters because you have more gameplay than a VN and your players hooked on a different aspect of the game.
I said the above assuming you are not fixed on Ren'py and maybe have some prior knowledge and experience with programming , but if that's not the case I think the systemic design is less flexible and sure, VN is the better choice IMO.