In my opinion, there are plusses and minuses to both.
On the king's side castle, the king is next to the h pawn, giving it some protection. The king is also one spot farther from the center. But on the flip side, the queen's rook is closer to the center with a queens-side castle.
But there are so many other aspects - like what your other pieces are doing, and how your opponent is attacking. How far into the game you are, and what open files you have. Plus many more.
I'd answer simply, there's no good answer - except "it depends".
To a beginner I'd give the advice: Don't be afraid to try either option. It might be to your advantage.
Anyone else?

When castling, is it better to castle on the king side or the queen side or doesn't it matter?