I vote for Venice too - at least you're in the same country - where are you landing? Venice can be seen in two days comfortably in my opinion. You would need more time in Paris - and with the flights and travel and all, you'd have LESS then two days in Paris and in my opinion, that's not even worth it.
In two days you can see St. Mark's Basilica and the Doge Palace, maybe the Accadamia and the Peggy Guggeheim Museum if you're an art fan. But the city's biggest attraction is the city itself. After strolling around and riding the water bus for two days, you will have pretty much covered the city.
I would advise Venice. You could fly in to Venice then proceed to Pisa. We love Venice, a small Place, but lots to see, 2 days is not enough. Take a Gondola ride towards to Rialto Bridge.