Re: aikido vs jiu-jitsu
Ooo this is an interesting discussion,
Well, I'm not experienced enough to discuss the techniques to apply in this situation but I can say you can use aikido. I think as a beginner we worry about this style, or this type of attack, or this environment, when in fundamentals, its all the same. In other words, its no different if he has a gun, or a knife, you have to use Aikido principles.
Some principles I think would apply: Moving where the attack is ineffective (at the far limit, or at the close limit), leading control (through atemi or what, again I'm no expert), maintain your center, keeping your hands in front of you, and blending.
As a beginner I don't think we'd know the best techniques to use in each situation, but we may do things that will keep us in the fight (or out).