I'm currently using Music Creator, but I've never used Sonar.  How do the features and operation of Sonar compare to Music Creator?  If someone knows how to use Music Creator would they be able to use Sonar easily, or would it require a lot of new study?  Is Sonar actually used in professional studios much?

MC6T is based on X2. Here is a comparison matrix between MC6T and X2 http://www.cakewalk.com/Documentation?product=Music%20Creator%206%20Touch&language=3&help=Comparison.html
 
The feature matrix for X3 is here http://www.cakewalk.com/Products/SONAR/Versions. With X3 the basic version has essentially the same features as Studio/Producer except for AudioSnap, ProChannel, SMPTE/MTC and Surround Sound support. Aside from those features the differences between versions are primarily the bundled plug-ins.
 

http://www.cakewalk.com/Products/SONAR/Versions
 
http://www.cakewalk.com/Products/Music-Creator/Features
 
There is no side by side comparison available but your question is best answered by you.
Are you desiring more features, more soft synths and options.
 
Music Creator is very close to what the older versions like Sonar 8.5 were in layout. The X series is a big change in how things are done and will require a lot of time on your behalf to re learn the software.
 
Best you download and try out the free demo first. Some of the Soft synths are missing in the demo I think, but you can at least see if the interface is better or worse for you. Give it a little time, don't make a judgement after only a few hours, at that point you'll be scratching your head, but there is lots of help on this form and no question is a dumb question.. 
 
The X3 versions all work the same but as you go up in price they add more Soft Synths and Plug ins. I would hazard a guess that Plain X3 is no better than what you already have. 
Make sure you are a registered user and ask for E mail notifications. This way you'll get offers to upgrade at way better pricing. I got Studio for only $99. It has more than I need so this is what you need to do. Look at the list of features and plug ins and see if there will be anything worth the upgrade price for. 
It's mostly about getting better soft synths for you. 

MC6 is almost like a cut down version of Sonar. It is a natural transition and would not feel alien at all.

Which version of MC are you using?
If an older version (not 6 or 6T) it will be drastically different, but not to the point you can't relearn a few new things. And it should work just as well.

To everybody, thanks for the comments.  I'm currently using Music Creator 6.  I found this screenshot on a blog here and most of it looks familiar (is this an accurate screenshot of the common layout?).  I think this answers my question.  I would have some more to learn, but I would find the environment familiar and I could probably get started quickly.  Thanks again.
 

Yes, X Series SONAR and MC6 share a common user interface. Here are a series of videos about SONAR http://www.cakewalk.com/CakeTV/SONAR-University/Get-Started You will find X3 similar to MC6.

The OP's question gave me a chuckle because I started my Cakewalk journey with Guitar Tracks, then began using Home Studio... when I asked a friend about SONAR, their reply was, "SONAR is like Home Studio on speed." I began using SONAR at SONAR 4 (PE).
 
For you, SONAR will do everything that MC does, just a "wee bit more" (understatement). SONAR will still import my Guitar Track projects (.wrk files!) without issues... the only "issue" is that very old effects need to be in the computer, and some 32-bit plugins/effects will not work in 64-bit SONAR. Up side is you can install both versions of SONAR (if necessary) on your machine, but I have stuck with 64-bit only. Opening an old project may give messages of "Plugin X (or Effect Y) not found" but the MIDI/Audio data is all there... you just need a comparable plugin/effect to replace the old with (and there are many bundled with SONAR). If you are running MC in 64-bit, this will be transparent to you since those effects are on your machine already.
 
Bottom line... get the SONAR X3 demo and give it a whirl... many things will be identical for you, but there is more you can do. If you are concerned of issues, create a manual restore point prior to the install, and you can always back out to that restore point if necessary. Stick with functions you are familiar with at first, and go from there.
 
This forum has a lot of very talented and experienced folks who give generously of their time to help others. If you hit a snag or have questions, simply ask a detailed question and you will often get a very prompt detailed response. I personally have not tried many others DAWs, and bought none of them, since this forum is by far the best resource to any SONAR user....

I started using Cakewalk products with Plasma, another (older) entry-level DAW, then Home Studio.  SONAR might look a tad different, but it is about the same as far as how you work.
 
As far as "pro studios," most use Protools.  However, many projects are started at home using whatever DAW the artist wants, and many acoustic projects are recorded at studios with Protools, nice rooms and mics yet mixed at home on your DAW of choice.  If you are looking for a job at a studio or production house, you'll need to learn PT.  But just like word processors, all DAWs basically do the same thing and learning any one of them helps you understand how to use all of them.
 
@