florence italy

It depends on what you hope to get out of your one day in Florence. Assuming no one in your party has been there before, and that you have the budget, I would recommend the walking tour. There is so much to see in Florence, and while it's pretty compact, it is easy to get turned around and lost. So, if your goal is to 'see' as much as you can in a very brief time, a guide would be very helpful. If you're more interested in just wandering about and enjoying the ambiance of the city, then being on your own is just fine. I, along with some friends, 'did' Florence in one day several years ago. We covered a lot of ground and only had a guide for the Uffizi (worth every euro!), but some of the group had already been to Florence before and even then, we didn't see everything we had hoped to.

I don't see how we could possibly answer your question for you. Only you know how much studying you will have done of the town, how much money you have, how comfortable you are in a city, a foreign city, what sort of things you want out of the city, if you have Rick Steves' book and can follow the superior walking tour in there, how well you can read a map, if you have IPods and Rick's or others' audio tours, how well you get on with guides, and a myriad of other criteria.
Only you know the answer.

Hope that your one day isn't on a Monday. Many major sites in Florence are closed on Mondays.

If you haven't been there before, then maybe a guided tour would be needed to see the major sites. If you don't want to do this, then you could walk on your own from The Duomo, to The Ponte Vechhio, and just stay in that area. It all depends on how you are arriving there? By car, by train, by tour bus?

Google "docent Florence Italy" and hire a guide, tell them what you want to see, and go for it.