I would completely recommend Francesca's apartment. It is only a minute or two from the Rialto Bridge but in a very quiet street and there is no noise at night - we slept really well.
The apartment is super clean and well equipped and laid out. As others have noted there are few stairs to climb but this is because it is at the top of a period building, which is why it is so quiet and allows for the fantastic sunny (even in March) roof terrace, with amazing views. The apartment is on two levels and the main bedroom and bathrooms are within the roof so for anyone very tall there are areas where the headroom is limited. It was not a problem for us (I'm 1.80cms).
Francesca was excellent at keeping in contact and always available by phone/email and her English is excellent. Her friend Sandra who meets you from the waterbus stop is also also charming and helpful. We can see why others have given this place 5 stars. You will not be disappointed if you book with Francesca!