So, If we are looking at it, roblox consists mostly of young people, so It would be easy to get them to your game with the cartoony style, but If you are not up to the cartoon style, to attract roblox’s young audience with the realistic style , you need to impress them, be ready to spend a lot of time on details in game, and to spend a lot of time on building.
Overall, it is your personal choice, if you are going with the cartoonish style, the the logical thing is for the rest of your game to be cartoonish, if going with realistic, then of course, the rest of your game would logically be realistic looking.
Considering that you are attempting to make something related to pirates, the story here could be mixed, my sense, honestly gives me that the cartoonish one looks better, it just gives me that feel, but at the same time, it is really hard for even me to decide, I suggest experimenting with the both styles, try making one portion of the game as you intended it to be, and then overlay both of the UIs, and then come to an conclusion which looks better.
Keep it also like this, [If going with the cartoonish style, the map, and probably the rest of the game should be made in the low-poly style, as most of the games keep it that way, if going with the realistic style, keep it real, make sure there is a lot of detail, the builds should be looking good and realistic. Long story short, if going with realistic, there will be a lot of work needed to be put, the realistic style to a game, always brings a lot of nitpicky people.