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Look around and see which is more active. Where I currently live the ACM seems dead and the IEEE seems pretty lively by comparison. I have been other places where it was the other way around. It more depends on who and how your local branch is being run than anything.

I don't see much use out of either other than as a line on a resume. With that in mind, I'd choose IEEE - more well known than ACM. (And if you're getting the type of membership providing access to research libraries, IEEE will cover a far broader range of topics than ACM).

I have been a member of both organizations for about 10 years now. I like ACM's magazines but IEEE's local chapter activities. So, it may depend on why you are joining.

Ask: ACM vs IEEE Membership