We will be arriving in Italy July 4/5 for 10 days. Proposed itinerary:
Milan
Florence or Siena
Cinque Terre
Venice
We will have 3 or 4 nights in Florence or Siena. The question is which we use as "home base". What is better Florence with a day trip to Siena or Siena with a day trip to Florence?
Any help would be appreciated.

Florence, while it may be the tourist epicenter of Italy, is still more comvient as a home base due to the amount of hotels, retaurants, and buses.

I see you've dropped Rome from your agenda.

The answer to this question, it seems to me, is whether you want to visit Florence or Tuscany. Florence is in Tuscany of course, but it's an essentially urban experience as opposed to the countryside experience. Staying in Florence, you'll not want a car. You'll find that the sites will be jammed with other tourists whether you base there or not.

OTOH, if you wish to visit some of the beautiful hilltowns of Tuscany, you should have a car and stay near to, but not in, Siena. You could day trip to Florence (car or bus) but, be prepared for the crowds.

(I think Florence is best visited during the off season when there are no unpleasant swarms of tourists.)

We did both towns in 4 nights and found that to be enough of all of it. I know, there is always more to see but you're just going to hit the high points on this trip.

I enjoyed Rome way more than anywhere else in Italy.

CT was a close 2nd but for very different reasons. I was so ready for some natural beauty and peaceful down time after Rome, Siena and Florence and CT gave me that.

I could go to Italy and never see Siena and Florence again and be happy.

Hi TuckH & everyone else,
Perhaps I should have said:
Rome or Florence or Siena on this thread.
I feel like a kid in a candy store who can only get one thing!