Paris Next September Milan or Amsterdam?

With 8 days after leaving Paris, you would have time to take in both Amsterdam and Berlin traveling by train.
If you headed to Italy, you'd do best to fly to Venice and take a train to Florence. Then backtrack to Milan by train to fly home. That's too complicated.
I'd go to Amsterdam and Berlin after Paris.

If you have never been to Paris before, I would spend at least a week there and fly home from Amsterdam, or if you want to do 3 cities, you could do Paris, Amsterdam, Berlin and fly from there. Paris needs more than 3 nights which is 2 days -- no fewer than 5 nights and a week is better; Amsterdam is lovely and a great 3 night city; Berlin like Paris needs 5 nights at least. So maybe 5-3-5 and home.

If you mean you're going to have to buy a one-way ticket home, I'd be comparing airfares and factoring that into my decision.

Unless this is a re-visit, three days is very short for Paris. Berlin is a sprawling city with lots of sights, many of them quite time-consuming (as in 4 hours, 6 hours, longer). Those two are more than I would want to tackle in 11 days unless Paris is a re-visit. However, I always spend a lot of time in large cities.

Unless you plan to fly to Berlin, you will loose a whole day getting there. As mentioned above, Berlin is a huge city and needs at a very minimum 6 days. Amsterdam has not appealed to me as much as destinations in Belgium, but that's just me. Belgium IMHO is frequently overlooked. Time wise, to me Paris to Amsterdam with stops in Belgium makes more sense.

You could actually stay longer in Paris. It is a blockbuster of sights, 3 dYs does not do it justice espec with jet lag upon arrival.
But betw Milan and Amsterdam I def would choose Ams. I was there in Sept. terrific stay. Hotels can be a bit pricey so book ahead. Enjoy. Great ambiance , Museums, canal cruise is a must. Walk alot and see alot. Gorgeous architecture.

If you fly into Milan Malpensa from Paris- its a short flight-you can take a small shuttle bus right outside the airport and be in the beautiful Lake Maggiore region of Northern Italy-it only takes a hour from Malpensa, you can see the Swiss Alps in one side, and the Italian on the other from Stresa- Mottorone Cable Car- its a beautiful area that I can highly recommend- you can take a train or shuttle back to Milan .

Hapoy Travels✨💫

I like the Paris to Benelux Idea.

Getting to Amsterdam on Thalys is super easy with many trains daily. It drop you city center. You have Anne Frank House/Museum, Van Gogh Museum, Rijksmuseum, Amstelking Museum. Tour a diamond shop. Take a boat ride on the canals ++++. Day trip to The Hague (Peace Palace by appointment), Harlaam, Aalsmeer flower auction is neat.

This same train makes a stop in Brussels 1 1/2 hours from Gare du Noord Paris, so you could add Belgium before The Netherlands. Brugge is a smaller town connecting in Brussels; museums, chocolates & beer.

Yep, Paris to Amsterdam via Thalys train is easypeasy. And yep, I'd definitely add more days to Paris if you've never been as there's oodles to do and see.

That said, we LOVED Belgium and it would be my choice over the other three. Have only covered Bruges, Ghent, Antwerp and a little of Brussels so far (Thalys to Brussels is even speedier than going to Amsterdam) but would go back in a heartbeat. Great architecture, art, history, beer (!!!!), food, friendly people, inexpensive country train system...

If you consider it, any chance you could fly home from Brussels instead of Amsterdam? It wasn't our favorite of the 4 listed but a good hub for your air out of the country.

One caution about Amsterdam in Sept? We flipped the order of a Paris/Amsterdam trip due to a big trade show: hotels are expensive there to begin with, and the show pushed prices through the roof + ate up a lot of rooms. Not that the same would be an issue on the dates you're considering but I'd check into what's on tap for 2019.