Xjapan, first go through the new FAQ section for keyboards to get a definition of different types of keyboards.
Next, the choice of midi controllers will depend on your needs. The main question is: what are you going to use it for? You plug it into Sonar, then what comes out the other end? This is crucial to your decision.
You have a choice of - hardware modules, VSTis, softsamplers, raw loops that you put into audio etc. Even if you are going to use VSTis, the actual choice of VSTi will determine the type of controller you need. The type of music you write is also important. This will determine whether you want to get a controller with:
1. More keys or less keys,
2. Weighted or unweighted
3. More knobs or not
4. More faders or not
5. Having different controllers on it or not (like X-Y controller, mod wheels, ribbon controller)
For example, somebody running analog VSTis like those from Arturia would want a controller with lots of knobs but not necessary many keys. Those in orchestral sampling (like me) will need an 88 key, must have mod wheel and/or faders, but not necessary knobs. After answering those questions, then look for the controller that best suit your needs in terms of functions.
If you still don't know, M-audio is the best way to go.