[ddots-l] Anybody have a crash course in midi controllers?

So I'm looking at possibly getting a midi controller, so far I've been looking into M-audio's stuff, the oxygen ofthe axiom. So why do I want one over the other? For instance, the axiom not only has more controls but different ones, it has knobs and sliders while IIRC the oxygen just has sliders. Does this matter? 

Basically I know nothing about midi controllers except:
1. They're keyboards. I want to upgrade and get velocity sensitivity and aftertouch and so forth that my cheap keyboard doesn't have.
2. I can use them to control parameters in sonar, e.g. do fades and the like.
That's about it. So I'm trying to figure out, do I want an oxygen for $150 or an axiom for $250 or whatever? I'm willing to wait and save up the money if the axiom just has stuff the oxygen doesn't, or if the extra controls are really going to make my life easier. So if anybody has any ideas for a complete newbie, I'd surely appreciate it. 
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