Why is that a question? Totally different shows, in my opinion. I think Arrow is awesome, but I enjoy both shows and feel no need to compare them.
I understand you're comparing comic show to comic show, but they aren't really the same kind of show at all. SHIELD is, to use my previous description, a mashup of NCIS and The A-Team that just happens to have superheroes, while Arrow is more like Batman meets Spenser for Hire (I know you remember that show). Nor are they competing, and there is no reason not to enjoy them both. Even if they were competing head-to-head, one is CW so it would probably survive much longer even if it were doing worse in the ratings.