Hi I have been taking aikido for about ten months. I would haved to say that if you trained hard for ten months you could obtain enough knowledge of aikido to use some techniques in an arrest situation. "Yonkyo" seems to be a favorite with police officers. And you night even learn some "knife take away".
But I am pretty sure you could say the same about judo. Within ten months of training you might learn some valuable choking techniques or "reaping" techniques. Both of which could subdue someone on the street.
I'd say that if you can picture yourself up close and wrestling with a criminal take judo. If you want to keep more distance between you and the criminal take aikido.