Hi Melanie, welcome aboard! It's a good problem to have: not being able to decide which of two amazing cruise options is best! Let me reassure you that either one will be magical!
You asked for my opinion in which of the two options I'd choose. Personally, I'd probably go ahead and choose the September 18th sailing so that my family could enjoy all of the regular fun onboard, but also experience the special events for Halloween! If you haven't already, be sure to read all about the spooky fun you can experience onboard the ships from mid-September through late October with the
Halloween on the High Seas cruises!
And don't worry about not celebrating your son's exact date of birth onboard. Disney Cruise Line is happy to help him celebrate a few weeks later too. When you book your cruise, be sure to let the agent know to make a note on your reservation that you'll be celebrating your son's big day - even if it's a bit late. Your dining servers will make a big deal out of it and sing him a birthday song and present him with a special treat one night. Additionally, stop by Guest Services to pick up a birthday button for him to wear. Cast Members all around the ship will go out of their way to make him feel very special!
So, that's my suggestion, Melanie! I like to experience as much as possible when I sail with Disney, so if going a couple weeks later means I get to have even more special fun, I do it! I hope that helps as you continue to decide! Have fun planning, Melanie! Stop back if you have other questions! We're happy to help!