Each skill has its own uses. Aim allows you to hit more often, so it has no comparison to Appearance which lowers attacking skill.
The way it is calculated is in order on your duel menu:
Everything is calculated with a dice roll. It will roll 1 to your current skill number. So, say, if your Appearance is 15, and so is your opponent's Tactics, it will roll a 15 sided dice, and whatever it lands on is compared to your opponents Tactics dice roll. If you have a higher roll, you get the advantage. The higher your skill points, the larger the dice becomes, and the higher the chance you have of hitting a higher a number.
Aim+Dodge: If the aim is higher, you score a hit. If the dodge is higher, you miss. In this explanation, the Aim will be higher so we can look at the other skills.
Shooting+Reflex/Vigor+Toughness: Depending on what you and your opponent are holding, depends on what skills you will use. If both of you are using melee weapons, shooting is greyed out and you will use toughness to defend. If both of you are using firearms, vigor is greyed out and you will use reflex to defend. If you're using different weapons, one of you will use vigor while the other uses shooting, the same as one of you will be using toughness while the other uses reflex. If Aim gets through, then it is up to the defensive skills to decide what you hit. Your offense will hit higher if your opponents defense is lower. You will score little damage if their defense is higher, though. This is a good determining factor of hitting criticals(or the highest amount of damage possible with a weapon).
Appearance+Tactics: These help intimidate your opponent. They are like a second attacking/defending skill factor, but are mainly decided at the start of a duel. As stated with the dice, depending on whose skill lands a higher number, decides the upper hand in the duel. A much higher Appearance or Tactics can make up for a lower toughness/reflex depending on the percentage it weakens your opponent. It can also lower a higher hit of the opponent to base damage instead of the critical he would have hit had the skill not been calculated. They are seen as scaring the opponent with such great skills that it weakens their own.
The ending result is how everything was calculated down to through the rolls. So if you roll higher than your opponent all four times, you have a good chance of landing one hell of a death blow.
I typed too much. Hope that helped.
