Well it all depends on if you want a dedicated control surface for Sonar Plat or if you want a midi keyboard to act as a control surface. I truly think the two are separate. I have several M-Audio keyboards for just using as midi keyboards, that's it. I have a Korg Workstation for using a workstation, that's it. However, I have a Mackie Control surface to use for only controlling Sonar X3e, that's it.
If you want a great control surface, see if you can find a used Mackie Control Surface, or even a BCF2000 from Behringer off of ebay or Craigslist.
Look here for a list of true control surfaces Check out the Behringer X-Touch compact.
http://www.sweetwater.com/c808--Mixing_Control_Surfaces