We can go to these three cities (3 nights in each) September 13-23 or September 20-30. Will there be that big of difference in:
1) crowds?
2) weather?
3) hotel prices?
I can't tell if I'm making the decision too big of a deal...

Topping...

I will be in Paris and Rome around the same times of year that you are considering. I would think that crowds are going to be similar in either time frame. I do know that September tends to be a big convention month for Paris. Also, there is the World Rugby Cup being partially held at Paris locations during the month of September, which adversely affected our ability to get Eurostar tickets from London to Paris for Sept. 14.
Weather is going to be similar for either - maybe a slight bit cooler for the second time frame, but it's weather so no one can really predict, of course. As for hotel pricing, I think it should be fine for either time frame, but I would go ahead and book something ASAP as September is such a busy month, particularly for Paris. Hope this helps!

We were told that September is still considered high season for most of europe so the hotel prices won't vary with your proposed dates; crowds will be moderate - conventions will still be in full swing but many families won't be on holiday then; weather - about the same either way. you can check weather.com and look at the averages for paris, etc in the month of september

you will have crowds but not as bad as the summer and the weather is normally very nice. check www.weather.com for averages. I've been to all those sites at that time and so far between September and November is my favorite time to travel.