Wiesbaden is on the same side of the river as Rudesheim, and has easy Autobahn or Bundestrasse (I forget which) access. Of course, the drive from Mainz is easy too; you can drive towards Bingen and take an auto-ferry to cross the Rhine which is kind of neat if you've never done it.

Another option is to get a taxi from your Mainz hotel to the Wiesbaden train station and hop on the train there.