Visiting watertowns from Suzhou

Hi,
I would like to visit one watertown while I am in the city for a day and then go back to Shanghai.
What is more convenient to visit: Tongli or Zhouzhuang since I want to go back to Shanghai from the town.
How long is the trip to any of these towns from SZ city and how to get there?
How long it will take me to go back to Shanghai?
Thank you