I would think a mid week visit mid September is probably one of the slowest times of the year and the weather is usually still warm and dry. If I could pick any time to go that would be my first pick. If you want to see the Halloween stuff I think it is up by late September.
Sept is a bit warmer on average which of course means Oct is cooler and maybe a little nicer. Oct it is more likely to rain. Sept in the first half has low crowds midweek but lots of refurbs happening. Refurbs are a bit less in late Sept and Oct. Oct has Halloween parties which take up evenings (this year at DL). But these may be fun to attend - if you pay extra. Oct has lots of special events. If you go on weekends then park hours and entertainment and crowds will not differ too much between Sept and Oct.