Which is more important in shogi, material advantage, defense, or attack?

It depends on the position and on how many pieces you have in your bucket. In many situations where a certain player has a material advantage but the opponent has a mate in a few moves because of the interesting movement of various shogi pieces. In fact many Tsume puzzles shows a mate or winning position with a huge material disadvantage.

I believe king safety is most important. A mated king is no good for the side playing with it.

Next I think is the attacking potential. Many factors are important and the following list, showing factors I have recognized, is certainly not complete.
-Material on the board and, more importantly, in hand.
-Position of the pieces. The farther up the board and closer to the enemy king, the better.
-Ability to 'sacrifice' material to get a piece you need. Example: Golds in hand are easier to mate with than horses on the board. The most valuable piece is the one you need/want most.

#4 Me too! Sometimes my opponent has more material with a exposed king. And I do, what I do the best! ;}

More important is to be balanced.
Attack when you must attack, defend when you must defend and go for material when you must go for material.

A few typical questions you should ask yourself:

Can my attack be successful with current material? If yes, then attack. If no, try to get more material.

Does a defending move give extra protection to my king/castle (thus forcing my opponent to waste more moves/material in order to crack my defense) or is it unnecessary?

Should I go for attack/grab material, or should I play a defending move/castling move/ king's evacuation move?

How many moves (check moves don't count) do I need until I can checkmate my opponent?

If I sacrifice material, can my opponent easily use that my material?

Does a trade give me better (easily usable) material or not?

Chess players typically go for material. Shogi is a very dynamic game, material advantage is important, but less compared to chess. In chess, a small material loss usually means game over, because the opponent will simply trade pieces and win an easy endgame. In shogi you must go for checkmate, so material advantage does not guarantee victory.

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