It's not professional to hire a person on the spot, or to give an indication that you're favoring someone. You could end up hiring your second choice because your first choice wants too much money or their references don't work out. It's good to call if you don't hear from someone, but I'd wait a day. If they say they'll let you know on Monday and they don't call, I'd call on Tuesday. It's also a good idea to ask, if they go with someone else, if they have any recommendations for you to be more successful in your next interview. Many people will just straight up tell you that the other person had more experience or "fit in" better. That can be really useful information.
_________________
I am a NT sociologist. I am studying the sociology of autism: Identity in ASD/AS, "passing" as NT, and causal effects of NT society on people with ASD/AS.