Would it be better to do a day trip to Cinque Terre or to Venice from Florence? We are not in Italy very long - 7 days including travel days) but would like to see as much as possible Planning on being there in mid September
Hate to be Debbie Downer, but I wouln't do either one. They are both at least two hours away by train, which means four hours round trip. Probably five considering getting to and from the train stations. You might want to consider Pisa, Lucca, or Siena instead. Those places are only half as far away, so you'd cut the travel time in half. Just something to consider.
Which would you rather do? A 3 (ish) hour train ride, including changes (each way) to see a series of small scenic seaside villages amongst a horde of your fellow day trippers. Or a 2+ hr train ride (each way) to see canals and historic buildings (and maybe some art) amongst a horde of your fellow day trippers? A lot of people who do these day trips don't like their experience at the end of the day; mainly because of crowds and feeling rushed. And they miss out on the very best part of staying in these places, which is the early mornings and evenings. So, it's up to you. Personally, if I couldn't at least overnight in those places, I'd pick a diffrent day trip from Florence. Perhaps Pisa and Lucca. Or Siena. Or a small group tour to the hillside villages.