Is it better to castle king side or queen side?

Let’s take a look at when it makes sense to castle opposite sides and when it doesn't.

You should castle on the opposite side when at least one of the following factors is true:

1. When you are up in development and your opponent has already castled, you should consider castling in the opposite side. That way you will have a clear game plan and will also be able to capitalize on your development advantage.
2. When you have a damaged pawn structure (doubled paws, missing pawns, far advanced pawns, etc.) on one of the sides you should consider castling on the other side.
3. When opponent’s pieces are especially active on one side of the board, it is usually best to castle on the opposite side.
4. If you want to complicate the game you may consider this option. That may be true if you must play for a win due to a tournament situation, when the draw is not enough. Also that maybe done when you're playing against a stronger opponent, who is much better in simple/technical positions. That maybe your best bet.

You should not castle on the opposite sides when at least one of the following factors in true:

1. When you are behind in development and you need extra time to develop your pieces, it is usually not a good idea to give your opponent a straight forward way of launching an attack.
2. When the opponent’s pawns are advanced towards the side you’re about to castle, it is not a good idea to castle there (especially if the opponent’s king is castled on the opposite side). It will just give him a positional edge in the attack.
3. When there are open/semi-open files in-front of the side you’re about to castle, you should probably reconsider your decision to castle there (especially if your opponent has castled on the other side). That will give him more attacking possibilities, such as rook lifts, various sacrifices, doubling of pieces on the file, etc.
4. If you playing against a weaker opponent you may want to avoid castling opposite sides, in order to avoid sharp game and keep everything under control.

Note: These are general rules, not laws, meaning that there are always exceptions to them. When you’re making a decision what side to castle you should always take your time and evaluate all “pros” and “cons” and base your decision upon your own analysis. This is a very important decision. It pretty much dictates which way the game will continue. Take your time and think twice.

King side castling is nearly always better
You can play O-O in 4 moves, while O-O-O needs 5 moves including a premature queen move.
The king is safe on the g-file, while on the c-file it is not and it needs an additional move to the b-file for safety and to protect the a-pawn.
O-O-O is better only when you plan to storm with your h-pawn, developing your king's rook on its initial square.