Well, I think Xitang is more charming and quieter than Wuzhen which has been commercialized. Depend on your favor.
From Hangzhou to Wuzhen: there are frequent buses departing from Hangzhou East Bus Station to Wuzhen directly. The fare is just 27RMB.
From Hangzhou to Xitang: only four buses depart from Hangzhou East Bus Station to Xitang directly everyday, respectively at 06:50, 09:25, 11:20 and 13:50. The fare is 38RMB.
But, the more popular way is that taking a bus from Hangzhou East Bus Station to Jiashan firstly and then transfer to Xitang quickly. The buses to Jiashan leave frequently from Hangzhou with a fare of 34RMB. When you arrive in Jiashan Bus Station, it is easy for you to catch a bus here to Xitang with a fare of 3RMB.

Hope this helps!