I agree Montauk can be excellent in Sept. Last Sept though the media finally got wind on the fishing and just stoked it up behond the pail. The crowds were sureal and it's no fun fishing shoulder to shoulder with a spinning rod let alone the long rod. Last year one Dip stick Fly Boy had actually shouldered his way into the line up and was trying to cast. Normaly this would have resulted in a Sharpie modifying his fly line before the idiot took someones eyes out.
I am back myself with a vengence this Oct for 17 days. In Oct of course it can get very windy and rough dirty seas can result. Tough time fly rodding but still doable on many days if you have the desire to do so.
I want to do the Cape myself in Sept after the big tides but it is too near my Oct trip to swing work wise. I have to make do with a May/June trip next year. I bet it's back to Schoolies then rather than the big fish I hear being caught this spring. But then that is fishing for you.