The two cities are very different.
Arezzo is a city of about 100K that is a modern city on the flat area near the train station and slowly merges into the older parts of the city as you progress up the hill. Near the top of the city is a grand piazza - featured in the movie Life Is Beautiful - and at the top is the cathedral and old Medici fortress now mostly a park. It is an easy walk from the train station although the very last part can be steep. The city has many events and actively schedules fairs or markets around the town or in the piazza and they medieval reenactment days with jousting so check the calendar for when you're there.
Lucca - the historic core - has about 9K people and is preserved within the thick historic walls. The surrounding area is more like 90K but tourists don't generally bother with the area outside the walls. Much of the city dates from the Renaissance and since the city was on the pilgrim trail there are many churches which are mostly used for other things now. Because the area inside the walls is considered fancier it is more expensive and has a lot of shopping and restaurants. The city core is easily walkable from the train station.
Lucca has become decidedly more touristy in the last few years and because it's smaller you'll feel the tourism more in Lucca than in Arezzo unless there is some big event in Arezzo. You can do Pisa and Lucca in a day if you're really only concentrating on the leaning tower and the Field of Miracles in Pisa. You don't mention the time of year but this area can be hot. Often tour groups that have come from Pisa are being dragged around Lucca by a guide but the people look like they want nothing more than some shade and drink.
Both cities have a range of dining including Michelin guide places. Arezzo seems to have little places on every alley way between the big streets that head up the hill. Places like Antica Osteria dell ‘Agania, and Antica Osteria dell ‘Agania are good and traditional. very near the piazza is Antica Bottega Toscana which has the real deal in terms of local produce - cheese still wrapped in hay or seeds - and is very much worth a look. They serve wine by the glass as well eats that go with wine as well.
In Lucca there are many good restaurants - Nearly any "famous" restaurant in Lucca - Osteria dal Manzo, Giglio - will probably require reservations a few days ahead. Piazza dell'Anfiteatro is beautiful and worth wandering through at night but is for tourists not locals. If you want to make that scene Sotto Sotto has the respect of the locals so I'd eat there.
I like Rosticceria Da Nonna Tati on the tiny Via Santa Zita - next to the laundromat in an old bakery location. They are doing new things with Italian food, but not crazy, and it's all really good but unusual given the adherence to "nonna's kitchen" in Tuscany.
If you want a place decorated liberally with penises - not kidding - there is Trattoria da Ubaldo.
Best sandwich place: Ciacco on P.za Napoleone. Also salads as big as your head - not kidding - excellent cicchetti as well.
Best gelato; Ele (formally De' Coltelli) and Momo in either location. There are other good places but these are the best - ele for wacky combos and amazing fruit flavors.
That's probably too much information, whatever you choose enjoy,
=Tod
