As Roberto pointed out, Pisa has a single interesting feature. Yes, there's the iconic leaning bell tower however...that's it! It's arguably the most overrated travel site in the world. Every two-bit tourist in the world is there taking idiotic pictures and every street vendor is there chasing all the tourists to hock their broken goods. Lucca is much, much more interesting, more places to eat, the surrounding wall is a great spot to have a picnic under a tree or, leisurely bike ride, many more shops to poke your head into, there's a couple of very cool wine bar/shops, it's history is extensive. Lucca even has it's own unique dish, Tortelli Lucchese and it's olive oil production is world renown. Lucca has it in spades IMHO.