king side or queen side

In general, your King is better protected in K-side castling because its closer to the corner.

But, condensing the excellent advice in Keres' and Kotov's The Art of the Middlegame's chapter on "Strategy and Tactics of Attack on the King" (the best 50 pages of chess literature I ever read), there are three reasons that give you an advantage or disadvantage if you castle Q-side and your opponent castles K-side (or vice-versa). 

1. Since opposite-side castling means attacking your opponent's King with a Pawn Storm, it's to your advantage to castle opposite if your attacking pawns are closer to his castle pawns than his attacking pawns are to your castle pawns.

2. Opposite castling is to your advantage if you have fewer of your own pieces to get out of the way of your attacking pawns than your opponent does with his pieces and attacking pawns.

3. Opposite castling is to your advantage if you have fewer of your own pieces to sitting in front of your castle pawns that lose tempos getting out of the way of his attacking pawns than your opponent does with his pieces and castle pawns.

In other words: can you storm his castle with pawns before he can do the same to you?

Note that when you both castle on the same side, attacks are most often made with Pieces, not Pawns, because the pawns are held back to protect the King - at least until late in the game.