This can be summed up very easily and quickly, it all comes down to needs and usage.
Almost every DAW does the same thing with the exception of a few percs here and there.
My advice is to use one and get good at it.
 
Time invested is worth more than the weight of gold.

All great comments... but the best is Craig's suggestion that you invest two weeks on each and then decide... it is SUCH a personal choice and what works for one human may not make sense to the other...there is soooo much I love about sonar but a few things are:
 

• The lifetime updates option with sonar, to me, is a huge issue and it will be taken off the platter by then end of this month, at least for awhile, I do believe...This frees up funds for other goodies...  
• The monthly rolling updates which cakewalk began a year ago have been stellar, quite amazing and for this alone, I think it tips the balance.  It allows for incremental learning and bug fixing, rollbacks are easy, and the bakers are able to respond to our desires more quickly than the traditional approach other DAW companies are still locked in.
• The track template implementation of Sonar is stellar and truly an amazing time saver... setups, adding them to a song as you are working on is instantaneous once you have a collection of templates you love, so this alone also tips the balance for me... it is sooooo powerful, especially when one considers the amount of time it takes to set up your multi-track Kontakt setups...I have DOZENS of them alone... you can have a slew of um at your beck and call... whenever you have a setup you like, save it as a new template and you can forever after bring it into any new composition AT ANY POINT IN THE SONG'S DEVELOPMENT...effects, tracks, submixes, etc... This ISN'T a SONG template, which is crude and sterile, which most DAWs offer...
• The ability to save multiple mixes and A/B them so easily is also huge to me...
• Finally, sonar's help systems, including the new pop-ups, online manuals, Craigs new and amazing collection of tips, and the forum... are all a big help in getting to grips with this VERY powerful program.  After many years I use but a fraction of Sonar, but as I need to do something new, it is there waiting for me...

 
...but ultimately it is up to YOU to decide which best speaks to you.  
 
Forum wise, you already know where you'll get the best help...
 
Best of luck, however you go...

I agree with the comments above, and add one more time that the Sonar forum is fantastic.
 
If you are working with singers, pitch correction is a deal breaker.
 
Sonar has a reasonable implementation of ARA for use with Melodyne. This allows more or less seamless use of Melodyne. The freebie Melodyne that comes with Sonar is not very good, but at least you can integrate Sonar with Melodyne. Just gonna cost you a bunch to buy Melodyne.
 
I am not sure about Cubase vocal pitch correction, so check it out. Last I looked they don't support ARA. But for all I know Cubase has something better. I just don't know.
 
Having said all of that, I have used Cakewalk since DOS 3. I think Sonar is a very good product, but it hurts every time I look at a vendor for add-ons to DAWs and Sonar is not on the list of supported DAWs. For instance, Slate Raven or Waves Tune. And it is frustrating when someone asks you what DAW you use, and then they shrug because they never heard of Sonar. Musicians seem to always recognize Pro Tools or Cubase. It would help if Cakewalk would do some lobbying with Waves and other big vendors to make sure Cakewalk is on the list of supported DAWs.
 
Just my two cents.

One of the other things, it could be a plus or a minus depending on your perspective, is Sonar is constantly improving by adding new features, content and fixing bugs.  However, you can always forgo installing the update each month and only install them when you see a feature you really want.

I fully agree with comments above, also check the list of VST instruments and plugins that come bundled with the DAW. I heard that Cubase give you very stripped version of plugins and you have to pay a lot for full versions later.