Late September or early October?

I'd go in September simply because you'll have more daylight and it's still late harvest time for apples and grapes in the countryside.

Go to RS Tours elsewhere on this web site and click on any tour that covers your area and look down on the lower right had side of the screen to where it says Tour Weather to get a feeling of average temperatures. As you perhaps already know, weather can change day by day however.

Riley, You might find it very helpful to attend the next meeting of the Rick Steves Denver group (sometimes referred to as the "Curmudgeon group"), which will be held in the next week or so. Check the other Thread in this section of the HelpLine regarding the meeting time and location. The knowledgeable and friendly group will be able to help you get all the details of your trip sorted. To answer your question, my preference would be late September. Happy travels!

I'd probably pick late September, possibly best overall month. October is great for Rome, though.

The October dates will have slightly less crowds, which, to us anyway, is a major plus.

Our last few trips have always done last 2 weeks of Sept and 1st week of Oct - (and not to jinx myself) but had perfect weather. And I didn't think crowds were that crazy - but I was left yearning for more daylight hours...that being said, I'd go mid-Sept (but prices could be lower as well starting Oct 1 if that's an issue).

Our preference is for Oct. The shorter daylight hours has little impact. The crowds are less, prices are better, and generally the weather is much better - cooler, dryer.