We are debating postponing our trip from Sept to November. What is the weather like then? Mild enough to tour around?

See weatherbase.com or worldclimate.com

What will you gain from the postponing?

Best wishes,

Rex

Airfare to Europe has just become so expensive (and we're flying from New York!) I was thinking it might be cheaper to go then, but judging by initial research it is actually not that much different.

Yes, I am frustrated by the lack of lower airfares in November - - or at least not so far this summer. Too soon, I guess. You may find sharper difference after mid-August.

Depending on where in POrtugal you are
going, you will find....warmish in the
Algarve but not swimming weather. Probably a light sweater or jacket would do.

It will be much cooler in the north and bear in mind that they will be off daylight savings time then and your hours for seeing things shortened.
November usuallly brings more rain then September (both in the N and S)....which can be very warm. I would definitely go in September if you can.

try using www.weatherunderground.com

You can select specific cities and then look at the weather history going back several years for a specific date.