Really it depends on the situation one is in.
But really you never want a street fight to go to the ground, especially if you're on top, and his friends are there. You'd receive several kicks to the head, if they were my friends.
So, really in a street fight / self defense you want the altercation to be kept standing, and end as quickly as possible. Therefore Karate does this the best. In my opinion. It teaches very good defense, while provides you with excellent striking and kicking.
Aikido, is good for some situations because if teaches standing submissions, and various control methods, but you'd have to be really really good with excellent coordiantion, otherwise you might miss your point of execution and not be able to properly perform the manuver, and thus leaving yourself wide open for an axe kicking.
Karate is the best in my world.
And thankfully have protected me in several situations that allows me to share my opinions today.
***There are guys who'd kill you over $5.00 all over the USA.
There are guys who want to fight just because, maybe even for initiation to a gang.
There are guys who want to fight because they are idiots.
And whenever one is placed in a situation that requires self defense it is a matter of life and death, for you never know whether or not you give up your wallet, and turn around and walk away, that this guy will break a bottle over your head.
Therefore, you must, keep your eyes on the person, and be prepared, and wait until they turn around and walk away.
"Fear No Evil, Trust No Man" author unknown