increase trip a night in siena/decrease a night in Cinque Terre?

I don't think there is a right or wrong answer...it is all a matter of preference for how you'd want to spend your time (since we're not talking about big cities like Rome that logistically need a certain number of nights). I'm not sure there is much to "do" in Cinque Terre, but that is the beauty of it...most people go there to just unwind, eat delicious food, hike, stroll, and relax. I don't think three nights and two days there would be too much by any means, particularly if you are trying to see all 5 towns. Two nights in Cinque Terre would mean only one day there, so you'd probably only see a couple towns unless you wanted to be on the go the whole time.

All that being said, if your priority is to drink wine, Tuscany is obviously the place to do that. And there are probably more options for things to go and see in that area if you are trying to drive out to different towns, so I might lean more towards more time there if that is your priority. In the end though, I think it comes down to how you want to choose to use your time. For either option (1 vs 2 full days in Cinque Terre, 2 vs. 3 days in Tuscany), you're not talking about huge amounts of time in either region, so both will probably be fairly quick tours (if you were asking about spending a full week in Cinque Terre, that might be different)...I wouldn't worry about feeling like you have too much time in either place).

I wouldn't change your plans. I like 3 nights in each. I would recommend taking boat in the Cinque Terre rather than the trains if the weather is nice. An all day pass on the boats was 24 Euros last year. Each of the towns are different enough, I don't think you would get bored. You can always arrive late in the day at the Cinque Terre to give you more time in Siena. During the month of September, the marble inlaid floors of the Duomo are completely exposed so you will be able to experience one of the most beautiful churches in Europe. Sounds like you will have a great trip whatever you decide.

To those who posted the two replies - i much appreciate it. my wife and i would like as many opinions as possible before we make a decision - anyone else who can voice their opinions as well, it would be much appreciated.

Three nights is way too few for Florence if you are actually interested in its artistic heritage; seeing that stuff takes time. Three nights is just two days. I would definitely drop to two in the CT. That gives you one full day for the trail which is a lovely way to spend a day. (We did this 35 years ago before RS unfortunately turned the area into a giant tourist trap; there were literally no other Americans there at all when we hiked the trail; the only other people on the trail all day were a class of Italian 14 year olds and their teacher on a field trip. There were no souvenir stands in the villages or crummy tourist food vendors. Ahhh the good old days) the CT is a good two night stop. We love Siena and often spend 2 or 3 nights there. It is a town that can be well served with one full day. I don't know if it is a good base for wine touring. I would think it would be if you have your own car. Otherwise florence is probably a better base for signing up for tours and you may want to be part of a wine tour as the drinking/driving laws in Italy are very strict and so wine tasting and driving don't mix if you are planning to swallow.

With the time you have I'd do 2 nights CT 5 nights Florence and 2 nights Siena. Or if you have a car (only useful for Tuscany a definite annoyance for the CT and Florence) do 2/4/3 and spend one full day out of Siena on wine visits.

Drop the third 5 Terre night and add it to Siena or Florence. Given your desires though it looks like Siena is your best bet. 5 Terre is great but one full day is enough to see all five towns and beautiful sunsets.

Jane makes a good point on the wine and other towns of Tuscany comment.
If you are renting a car Siena may be a better base than Florence for experiencing these things but if you are not you will find many more tour options and transportation options out of Florence than you will out of Siena.
Siena is the place you may want to cut back to 2 nights, giving Florence 4 nights and keeping Cinque Terre 3 adding a day trip out of Florence to the plans.
Or even Florence 5 and the other 2 each ; adding 2 day trips while in Florence.
If you renting a car it does not make too much difference Florence or Siena time wise but usually Siena driving/parking will be easier depending on where you are staying.

I suggest you stick with 3 nights in each place. My husband and I visited those same places (among others) and except for Florence where we stayed 3 nights, we felt short-changed by only doing 2 nights in the CT and 1 in Siena (which was just plain dumb on our part). We're big wine people, but found that it was just as easy (and enjoyable) to experience Italy's great regional offerings in the CT as it was in Florence and Siena. And you will just love spending full days in a place (ie, going to bed in the same place you got up that morning) instead of dealing with packing and logistics of moving.

thank you everyone for all your replies. yes, we are renting a car the day we depart florence for siena.

We are staying at the Aia Mattonata Relais in the Siena area for three nights, and then driving from there to La Spezia (dropping off rental car there), and staying at La Cabana for three nights in Cinque Terre.

Based on comments, I am still torn. Still want to stay in Cinque Terre for three nights, but I really think we'll kick ourselves after the trip if we don't add a night in Siena/take a way a night in Cinque Terre. So I am leaning toward that - if we do, we're going to leave Siena early on the day of checkout and get to Cinque Terre in the morning hours so we can explore Cinque Terr/relax there for most of the day (and just leave our luggage at the hotel until we can check in).

That's what I'm leaning toward anyway.

Keep it as is. Cinque Terre has wonderful wines also. We were there last September and it was grape harvesting time. The gentleman running the affittacamere we stayed at brought a bunch of grapes to our room after we checked in. They were delicious, still warm from the sun. There's a wonderful outdoor tapas/wine bar in Manarola located up the path and overlooking the water. Excellent local wines. I have been to all three locations you're visiting. They're are all equally amazing. Regarding being wine fans - you'll be in Italy, no matter where you go, you'll find fabulous wine.