Modular buying strategy?

I ordered Guitar Tracks Pro 4, should be here Monday. I'm pretty excited, but I want some of the functions in Sonar X1 Studio, but I just couldn't spend $200 blind without knowing whether I can get it to work for me. I knew that Guitar Tracks has almost everything I want. What is missing is V-Vocal, and the Synth rack, plus Skylight.

As I understand it, the Skylight and other interface features are simply updated ways of displaying the various views that require toggling in GTP and older Sonar maybe? But functionally, what am I missing between GTP4 and Sonar X1 studio aside from what I mentioned? Can I buy some of these as modules?

I'll be recording guitar and MIDI keyboard plus vocals. GTP4 can handle that and it looks like the Virtual instruments are pretty good. If I do get good use from it, I'm going to have to decide how to deal with the missing features I want. I figured for $80 I can get started and learn enough about Cakewalk tools so that I probably will understand better what I am missing without Sonar.

So, if someone can offer some insight in what to expect, with GTP4 and how to get the features from X1 Studio of I decide I can't live without V-Vocal, is it even sold separately? I guess I should see if I qualify for upgrade pricing. My last question is; will it have been wasted to buy GTP4 if I end up buying X1, or does GTP4 have a different set of tools not available in Sonar X1? I wish Cakewalk would just offer a single platform and then offer modules, so maybe in my case I could get the "starter" X1 with a guitar oriented add-on pack. In essence, I think that is what GTP4 is, BUT it seems like the X1 interface is a newer generation? Or am I getting that in GTP4, but they don't happen to mention?

Still a little confused.

Thanks, Chris