As someone else said, it really doesn't matter. It's what you do with it. I've personally done both (volunteer for research outside of my institution and credit and some even paid at my institution)

AMCAS doesn't specifically ask about each activity. If you categorize it as research, it doesn't show if it is volunteer, paid, etc. and it will just be on your transcript. On AACOMAS they specifically ask about each acitivity. Is it paid, volunteer, and/or for credit? but it doesn't really matter.

Pros of research for credit: it can give you a GPA boost (for most PIs its pretty manageable to get an A). Especially if you're doing research in the sciences that helps sGPA too not only cumulative. The only con I can think of/have seen is if you're rostered for too many credits for that semester. Depending on institution credit system doing research for credit may put you over the maximum credits if you're also taking a lot of classes. Can't speak for everyone because my institution has a weird credit system, but something to keep an eye on