For starters, X1/X2/X3 are previous version, and are no longer available.
Sonar Artist/Professional/Platinum are the currently available versions. They share the same core program, but Professional and Platinum have more toys. Se the comparison table Wookie linked to above.
If money is not an issue, Platinum is certainly a great package. However, my guess is that the vast majority of Platinum owners will never use more than 30% of what the program is capable of.
Heck, I have Professional, and it has stuff I may never use. That said, Professional is a worthwhile jump from Artist.
The real entry level Sonar is Music Creator. This is not universally understood.
It still has a fairly fierce learning curve, but many functions have been simplified for new users.
The downside is that there is currently no upgrade path from Music Creator to Sonar as there was in the past.
You mentioned that you will be mostly using VST's.
Will you be using a controller to record your music real-time? If so, start shopping for an interface. With VST soft synths, you are entirely at the mercy of your sound card/interface, and latency is the first hurdle new users encounter.