That is a tough one but I think I would pick 16 months. By then they will be able to run around and play in the play areas. We just took DS (18m) and it was my favorite trip of all time. Watching him interacting and playing was so much fun. He also really enjoyed the pool.
At 11 months my DS wanted to crawl everywhere and I was really grossed out at the idea of him crawling in a hotel room or public. The older they get the easier they are too feed (unless exclusively nursing), no worries about formula, milk, baby food, etc.
In my experience May is hotter and more crowded than Oct. When we went in May there were tons of preteens and teens on school trips making the park more crowded. I would pick Oct over May if I was planning a trip.
I honestly don't think you can go wrong either way. I am sure you will have a wonderful trip no matter what you choose!