X2 comparison PDF X2 Essential/X2 Studio/X2 Producer:
http://www.cakewalk.com/products/SONAR/SONAR-X2-comp-chart.pdf Get what you can afford you can always upgrade to next level up later on.
I bought X1 Producer when I had the cash and there was a sale, it's everything in the box. I bought the X2 upgrade cheap during a sale - but I don't have it installed or use X2. When/if X3 comes out, I would probably be fine with X3 Studio as I don't tend to use any Cakewalk synths anymore and most of my effects are 3rd party.
To each their own. Only you can decide what you need/can live with/without and how much money you have.