When the Swordsman levels up, every other time (until about level 15) he is given a choice of increasing his strength by 1, his vitality by 1, or his stamina (health points) by 5. To answer your question, we need to consider what each does.
When the player's strength gets above 15, the extra points are added to the damage his weapon can do. Let's say the player holds a weapon which can do 30 damage points to a monster. If the player's strength is 25, that same weapon will do 40 points of damage (10 points above the 15 are added to the weapon.) You can buy potions for strength later in the game.
Vitality is a little harder to see because it only affects the player on level ups. If the vitality is over 15 points, an additional amount is added to the stamina on every level up. Vitality can also be purchased but it comes at a point where level ups are fewer and farther between.
Obviously, with more stamina you can last longer in battle. You cannot buy stamina potions.
To many players, strength may be the first choice, but I value stamina and vitality more.
My strategy would be to increase the stamina the first couple of times, then get the vitality up and worry about the strength later in the game.