Probability advice for a rpg

Hello everyone !
The question I have will look a lot like a math problem, and I have to say, if I was a lath teacher, I would use problems like these all the time, it's way more entertaining :
I am playing a role-playing game set in the star wars universe. For those who know star wars, you know that jedi use their lightsabers all the time to deflect shots fired at them, and of course it's possible in this game, but there are two possibilities, and rhis is how it works :

1 - The ennemy fires at you. He rolls a 20 sided dice (which we refer to as d20), add the bonuses he may have, if it's more than your defense, you get shot.
2- if you're a jedi, you have the possibility to roll a d20 too, and if it's more than the result of the attack, you deflect the shot.
3- Now imagine there's a second opponent, who fires at you at the same time. You repeat the same process, but when the jedi rolls his d20 to deflect, he has a -5 malus. If there's a third, he gets a -10, etc. Note that while this increase every time, each malus only applies to one roll (so if you deflect three times, the first roll would be with no malus, the second with -5, third with -10)

Now when you get stronger, you can improve that in two ways, and that's the heart of the question. You can either :
- roll two d20 for each roll, keep the best, but other than that every thing works the same.
- only roll one d20 for each deflect, but the penalty only increases by two for each shot instead of -5 (so in our first example, no penalty for the first shot, then -2, then -4)

My question is simple : what's best ? I don't know much about probabilities but I suspect the first one is better if only one ennemy fires at you, and the second is better if more than a certain amount of ennemies fire at you, but I'm not very good at this, so I may be wrong.

Anyway, thanks a lot in advance !