A bit more background--

Most of the forum threads I've looked up about Siena versus Lucca are by posters with only one day to spare, and most if not all have been recommended Siena because of the architecture and history. I, on the other hand, have two extra days, and am also conveniently headed West after Florence, so can equally consider Lucca and Siena as part of my trip (and will want to spend the one night, at least, in Lucca).

I'd suggest Lucca and Siena. Lucca is fantastic, but it is eminently doable in a single day. If I remember correctly, I walked the entire wall in just over an hour and I remember climbing to the top of a tower (Duomo or watchtower??) which was good for maybe 40 minutes.

Having visited both places several times I would choose Siena for two days.

Lucca and Siena are two of my favourite cities in Italy, but in terms of sights and places to visit, Siena is vastly superior.

I'd suggest one or the other - either are equally attractive to me - but not both. 2 nights does not equal 2 full days, it equals 1 full day plus a couple of bits of days. Out of interest, how long are you spending in Florence? We went for a week last year (our 2nd visit) & even though we don't like it much, a week wasn't long enough to see all we'd hoped in the city itself.

We are looking at a day trip by train ex Florence, taking in Pisa and Lucca, then back to Florence.
Perhaps that is an option to consider - one day in Pisa, then on to Lucca in the late afternoon / evening, and spending the second day there?

You can easily see the highlights of both Pisa and Lucca in a day, a daytrip sounds a good idea. Lucca is a nice town to be in but not alot of sights, as everyone above has said. But you have not told us how long you have in Florence. Like Caroline, it is not my favourite city but there is an immense amount to see.

I'm not a fan of Lucca so I would go spend the two nights in Siena.

You could easily day trip to either town from Florence if you just want to stay put those two nights. You can bus (Lazzi) or train to Lucca and then go onto to Pisa (20 minutes away by train). You will have more return options to Florence later in the day from Pisa.

You can also bus (Sita) or train to Siena (long series of escalators now available from the train station up to the center). Both take a about 1 - 1 1/2 hours.

Either day trip will cost about 7€ each way.

You don't mention how you'll be getting around so I'll describe what we did.

We took a train from Florence to Pisa but not to the Central station. We got off at the next stop (make sure your train stops there) which is Santa Rossore. It's very small and only about a half dozen of us got off there.

We didn't even go to the little building across the tracks. I had printed a little map. This station is less than 10 minutes from the Leaning Tower area.

We spent about an hour around the Leaning Tower and then went back to that same little station in time for the next train to Lucca. This was around noon time.

A few minutes later we were in Lucca. We walked right thru the town, wandered around some squares and walked along the mail pedestrian street. When we got to the far end of the town we pick up rental bikes and spent about an hour riding on the wall around the town. We stopped for a lunch picnic which we brought with us.

Then on the way back towards the train station we made a few tourist stops (I remember going up one or two towers - photos at www.travelwalks.com) and my wife picked up something she had seen in a store on the way.

From Lucca we took the train to La Spezia. Our plan was to keep our hotel in Florence and sleep over in La Spezia taking with small backpacks with toiletries and a clothing change.

We got to La Spezia in time to check into our hotel, which was located less than a block from the train station, and take an 8 minute train to Riomaggiore to see the spectacular sunset.

Then we ate at an outdoor restaurant before returning to La Spezia for the night.

The next day we trained back to Riomaggiore, hiked between a few of the villages and took a boat to Portovenere and eventually after wandering around there a bus back to La Spezia to pick up the backpacks from the hotel and train back to Florence.

Sounds hectic but it was a very relaxed and excellent two days. The important thing is knowing where each place is located and how to get there. Train and boat schedules are also good to have. So we spent a minimum of time figuring out what to do and how and maximum time enjoyng the places and the views.

I think that Siena has more to offer and is more spectacular architecturally. I would spent two days there rather than in Lucca.

http://www.flickr.com/photos/mksfca/sets/72157622914855341/show/