Being obviously partial towards Halo, I think my answer is fairly obvious.
In an attempt to be more objective, I will say this:
The MCC is a great game with flaws that can hinder certain experiences. I like doing campaign stuff, so the more prevalent errors in the MP aren't as impacting on me. Necessary to say, the MP is not up to previous standards in speed and accuracy, but it is improving.
My more objective opinion is to get the MCC if you don't mind the slightly longer search times. Otherwise, get GTAV, and then get the MCC for the holidays. That way, when you do get the MCC, it'll be more fixed than what it is now, and you'll have it in time for the Halo 5 Guardians Beta.