I'm planning a trip for 2020 to London/Paris. I'm looking at the following travel dates; September 13-26 or October 4-17. Which would be best?
Best for what? Prices, fewer tourists, weather?
I can’t see why one set of dates would be better than the other, frankly. Are you worrying about the weather? Or the possible crowds? What suits you “would be best”.
October, wetter, colder and shorter days.
Better weather, shorter lines, better prices....
13th September: sunrise: 06:31, sunset 19:20, daylight 12 hours 48 minutes
4th October: sunrise 07:05, sunset 18:32, daylight 11 hours 26 minutes.
A whole hour less of daylight! (source: https://www.timeanddate.com/sun/uk/london )
October will have slightly less tourist, so lesser queues. It probably doesn't affect prices much. Weather colder and wetter.
I'd pick October for fewer crowds, cooler weather, etc. Sunset is still after 6:00 PM, and frankly, particularly in the city, I don't care how early the sun sets. Life continues after sunset.
So, weather may be marginally better in September, and the daylight will last a bit longer, if either of these makes a big difference for you. See no reason for prices to be less and queues to be shorter in late September than in early October. You are contemplating the two most visited cities in the world, so there will be crowds and few bargains. But ........so worth it.
October.
Will still be crowded but not the massive hordes of Summer.
Book your Eurostar tickets in advance to save money.
Double check when the Nuit Blanche will take place in Paris. This year it's on Oct. 6th
What is the Nuit Blanche?
In French " white night "
https://en.parisinfo.com/paris-show-exhibition/135251/nuit-blanche-in-paris
EDIT: and I'm confused because on your other post you've already set forth a September travel itinerary.
Are you still debating which month to travel?
I would also decide based on exhibits or events that might occur one or the other weeks....though it might be too early to see what is happening 18 months from now.
We love September in London. The weather is still warm, evenings are nice, and there is a month-long festival celebrating the Thames, with events and installations.