a sunfish is a small boat, these are great boats. What type of sailing are you going to do? Around the markers? Cruising and such?
Both sail well, perhaps a few points to the First, as it is a First (the racing series, as opposed to the Oceanis). I have owned both maker's boats at one time and can say that, either will serve you well. If you lean more to cruising, I would opt for the space and comfort of the 30i, more to racing then the First.
Either boat of that vintage may need some repairs, so make sure to get a survey and be prepared to replace some bits and pieces.