I think different circumstances affect the answer. In a vacuum, I say getting certified at the beginning of the season is better just because you do have so much of diving season left, but for your situation getting certified sooner (Sept) is more beneficial for a couple of reasons:
1. You are already on this site and have 14 posts. Even in a wetsuit you can dive until the end of Oct (that's 4-6 weeks of diving). Reading and research is one thing, but there is no substitute for actual experience. You can get certified, gain some real world experience, and then ask questions and do research on this site with a MUCH better grasp of things
2. Financially, scuba is never cheap. Getting certified in September gives you time to save money and research what gear to buy in the off season before diving beings in April/May.
3. Scuba is a continuous learning exercise. There are a lot of different courses (Night, Nav, Nitrox, etc) that you will taken after getting certified. I wouldn't put the highest emphasis on "losing" skills in the off season, review your notes, take it slow. You would be surprised what comes back to you. I had over 200 dives at the beginning of this season, first dive back after not diving for 4 months I felt like a complete mess.; a couple of dives and you will be back to your old form.
PS...Are you going to be diving in a local quarry? A bunch of places of specials where if you get certified in Sept/Oct and buy a season pass, the pass will be good for all of the next dive season....saves a little money and gives you another reason to dive.