What is justice ? Shall we seek righteousness or shall we seek power? Is it better to be good or to be strong?

According to Merriam-Webster, justice is the maintenance or administration of what is just especially by the impartial adjustment of conflicting claims or the assignment of merited rewards or punishments.

The second part of your question calls for your opinion. Sun Tzu says that....

“Men proficient in battle do not easily grow angry or lightly instigate war....”

“War is not a pastime, no mere passion for venturing and winning. It is a serious means for a serious object.”

From this, we can infer that he believed righteousness is more important than power, as power can be found in a strong military and powerful warriors, who use their power in a righteous manner.... not to prove how powerful they are, but rather, reasonably and for a justifiable cause.