September or October to visit Venice, Florence and Rome.

The crowds will be similar, the weather is unpredictable, except for just days before when weather forecast become more reliable, temperatures will not be significantly different (statistically it could be a degree or two cooler later in October, but nobody knows for sure). The only sure thing is that the earlier you go the longer the sunlight will be.

Fall could arrive at any time during that period, bringing with it coolish temperatures (especially in Venice) and rain. I don't like that kind of weather, so I would choose your earlier option. I was just in Venice September 17-27 and was very happy with the weather; however, weather can vary a lot from one year to the next. It's unlikely you'd run into really cold weather by waiting two weeks; I think the major concern would be rain. I don't know whether acqua alta is at all likely in Venice as early as October.

What I'd do to check on lodging rates is use booking.com. I'd choose a few hotels in my approximate price/quality range and compare their rates for the early and later time periods. That should be revealing.

I haven't been in Italy in October for many decades (if ever), so I don't know whether there is likely to be much difference in tourist crowds over the period you're focusing on. I'd think any special events could overwhelm general drop-offs in visitor levels, and traffic in Venice would be affected by the number of hulking cruise ships docked in Ravenna, etc., and busing folks to Venice.

It was very hot our first time in Rome which fell within your September dates. I recommend October.

Roberto's point about the hours of daylight is a good one. At this time of year, I become very aware that the days are rapidly getting shorter. It really, really bugs me; I know it's not much of an issue for most folks.

Higher chance of cooler weather in October, so that's what I'd pick for sure. No difference in crowds or costs.

Our preference for travel in that area is Oct especially the first two or three week and maybe the last week of September. The will be min differences if any in your time slots. Those three cities are always busy so really doubt if you are anyone would notice any decline in number of tourists. Weather should be great and cooling some but heat spells can occur at any time. There will be no cost difference. I personally would pick your later dates.

I did Venice/Florence/Rome back in 2015 last week of September, first week of October. I chose that time so it would be a little cooler. The weather was mostly nice, no jacket needed during the day, but a light jacket in the evening. It did rain in Florence mid trip, and then needed a warmer layer and jacket with hood. In Rome I got caught in a flash thunder storm, and it rained so hard I just found a cafe and had lunch. Then the weather cleared and I had to stuff my wet jacket in my backpack. I don’t think there will be a difference in the crowds, and the two weeks won’t make much difference in the weather, it just might be a bit more changeable.

We visited Mid/late September (Florence, Lake Como, Venice) about 5 years ago and the weather for us was glorious. The crowds in Venice were awful but with the cruise ships I think its hard to avoid crowds but the evening in Venice were great.

I like to look at the cruise ship impact for Venice. I believe later is better. And, I check local holidays for each location

Flip a coin.

Just got back from Venice and Florence and I can guarantee the crowds are still in full effect in September.
October will be a bit more "shoulder season" than September but really with 2 weeks between them it's probably just rolling the dice as to which time happens to have the better weather.
I'd say just pick one and go with it.

Have a great trip,
=Tod

Our first trip to Italy was in October 2010. We arrived on the 4th and spent 2 weeks. We had two days of rainy — not deluge — weather. We often reflect on that trip as we are usually in Europe in October.

Since that first trip, we lived in Italy for several years and October was my favorite month, followed by April. Seldom need jackets and when you do, a rain jacket is the best choice. Umbrella sellers will pop up out of nowhere if rain is coming and you can grab a cheap umbrella if you feel you need more protection suddenly.

Travelers will be fewer, for the most part. The Vatican will still be a zoo. Make timed entry reservations everywhere you want to go but allow time for walks. Rick Steves has great self-guided walking tours in his books.

To me it comes down to how heat tolerant you are. I don’t see your location so impossible to guess. I live in N.Idaho and anything above 75 is hot and uncomfortable to me. I was in Venice for a couple days last week (Oct 4/5/6) and it was pretty hot in the afternoon for me and absolutely stifling in the Doges Palace. Mosquitoes were bad at night so needed to keep the windows closed and hotel AC was off for the season.

Missed rain in Venice but got a deluge yesterday AM leaving Cinque Terre.

We live in South Georgia and anything below 80 is wonderful.

If you like cooler weather do the later trip. I would pick the earlier one, since I don't care for cold weather. In any event, the weather is likely to be moderate.

Which trip is more expensive?

We just got home yesterday from a September 9th to October 9th trip to Italy (basically Rome to Genoa staying in hill towns and towns in the hills) and swore that we'd never go to Italy in September again --- too hot for us, especially for me. It was our 3rd "boiling hot" September in Italy. We like cold and fewer people and even some rain and fog. Better to wear a sweater than to BE a sweater. But, that's us.

What might be relevant to you is that far fewer restaurants had outdoor seating in October --- I had been hoping to avoid eating indoors in small crowded rooms ("small and crowded" would be a partial description of the kind of places we preferred to eat at pre-COVID). In weather that, to a Minnesotan, is still quite hot (70's), a number of restaurants that we chose with "outdoor seating" had everybody inside with all the doors and windows shut, even though the tables with umbrellas and the terraces had been in use a few days before.

As for tourists, there will be very little difference between your two proposed dates.

Thanks to everyone for the great advice!! We are from Iowa and prefer the warmer (even hot) weather. Our plan is to see the major sites while eating and drinking our way through Italy, so the outdoor dining is a must.