I would take the internship if possible but don't sweat it if you don't. The time at the airline can help but it can definitely hurt. If you ever have a run in with anyone, it can come back to bite you in the future. Same with time at Horizon. I have personally read a letter from a past chief pilot to HR that told of a specific pilot at Horizon and to not offer him a job.
Also, I know of two former interns that are still waiting for the "promised" interview. They will get one but they don't seem to be on the top of the list.