MP 15-22 advice: Should I buy a scope or red dot sight?

Depends on what you want to do
red dot is great for short range quick acquisition
scope is better for precision at longer ranges

on mine I put a Nikon pro staff on quick release rings so I can go back and forth from irons to scope

I have a red dot on my 15-22. Extremely fun to shoot shoot quickly at multiple targets.

I vote magnified like a scope.

I see a red dot sight as purely a quick-acquisition type sight. So in that context you really have two different reasons for them:

1. Self defense (close range - like my home defense rifle)

2. Competition

I can definitely agree that red dots are super fun for plinking so that could be a third option but I still think a magnified optic gives you more versatility there.

I'd say ideal for a 22 that I may plink/hunt with would be a 2-7 scope with like a 30mm objective. I like this option from Redfield (2-7x by 33mm) $176:

https://www.amazon.com/Redfield-67085-Revolution-2-7x33mm-Accu-Range/dp/B0034L8XW4/ref=sr_1_2?s=hunting-fishing&ie=UTF8&qid=1477254941&sr=1-2&keywords=redfield+scope

I put a tasco red dot on mine. I like it, it was cheap. Works great on a .22. Amazon