Train and bus are both convenient,here is the information I got on this website:
By Train:
Fifteen trains run between Hangzhou and Suzhou daily. Seven of these depart from the Hangzhou Railway Station (which runs between 08:42 and 18:44). The Bullet Train takes 2 hours and 40 minutes to reach Suzhou. The other eight trains depart from the Hangzhou South Railway Station, which runs between 01:44 and 19:44. Sixteen trains run between Suzhou and Hangzhou, among which there are two Bullet Trains. Bullet Train tickets cost between CNY89—CNY107.

By Long Distance Bus
Passengers can take long distance buses from either Hangzhou Passenger Transport Central Station or Hangzhou North Bus Station. About 16 buses run to Suzhou from the Central Station daily between 08:00 and 18:30. Buses from the Central Station arrive at the Suzhou South Bus Station. The two-hour trip costs CNY71. More than 20 buses run between the Hangzhou North Bus Station and Suzhou North Bus Station from 07:00 to 18:30.