Regional is always a better choice no matter you want to apply for cx or not because Ameriflight simply isn't something you want (at least myself) or will give you any bigger advantage in a long run. thus, what's next on the list after Ameriflight? possibly regional if you want some jet time (i am not saying that Ameriflght doesn't have any jet, you know what I mean) or some real airlines operation experience.
another reason is if you failed on cx or other major airlines, regional can still be a life time career in additional to salary and benefits
after all, this is my just opinion and what I would do if i am in your position