Looking for opinions on Sonar soft synth voices vs other commercially available synths. I have Sound Tank but haven't invested any money in any sounds yet. Just the included freebies. The sounds that come with Sonar seem pretty good. Not sure which direction to go. Advantage to Cakewalk being guaranteed no compatibility issues.

It might be helpful to list the Sonar version that you have, as some versions provide a good set of sound content out of the box.  Then some suggestions could be made to fill any gaps ...

opie1 . . . also, let us know what kind of music you're shooting for. The synth / sample sounds you'll need will vary depending what you're doing . . . eg. EDM, Orchestral, Pop, Jazz, Rap, R&B, Folk (to name a few) etc . . .
 
Cakewalk's synth offerings cover a lot of range, and are certainly enough to get anyone started, but there are always higher end (more expensive) soft synths / sampled instruments available depending on what your genre requires.
 
I wouldn't worry much about compatibility with Sonar, it supports most VST synths you throw at it.

Just installed Sonar Home Studio. I haven't tried the sounds out yet. My experience has been with the older version of Home Studio I was using. I thought the sounds were pretty good that came with it. Music genre is a tough one, 60-70s classic rock, classical and a little folk music. A lot depends on what my fingers and brain feel like doing. I can more easily say what I don't do; techno, ear pounding beats, rap, disco, modern pop, and probably more. I guess my wish is to keep my new setup simple, with max sounds (that I will actually use), at reasonable cost. I may have answered my own question. Will I actually use all the sounds that come with a 500 soft synth?

Is it keyboard sounds you're looking for or are you creating everything "in the box"? For keyboards I suppose you'd need mainly piano and some organs (Hammond, Vox, Farfisa).

To me it seems a bit useless to outline all possible synths and libraries you COULD buy just because they exist. I'd recommend trying to make a song and asking for suggestions if you can't find or don't like a specific sound.

Hi.SONAR Home Studio comes with Rapture Session, which includes quite a bit of different presets / styles / instruments.
I would check them out, before going any further.
You just might find what you need there.
All the best,

The bundled synths are excellent, but most are oriented toward "modern" styles. Only Dimension Pro and TTS-1 offer the kinds of sounds you're looking for in classic rock and classical music (e.g. strings, piano, percussion). For your genre, consider adding Kontakt from Native Instruments. It's a significant investment, but once you have it it will soon become your primary software instrument.

Between the free Kontakt Player factory library, and the Sampletank free (Custom Shop) factory library, you can get some useful instruments right off the bat for free.
 
In the Kontakt Player Band section, you get a few basses, electric piano, jazz guitar, jazz organ, rock guitar, and a few drum kits.  There are also some vintage, synth, and world sounds.  If you upgrade to the full version, you will have endless opportunities to buy more expansions.  Some expansions are even available for the Player version, without requiring the purchase of the full Kontakt version.
https://www.native-instruments.com/en/products/komplete/samplers/kontakt-5-player/kontakt-factory-selection/
 
In Sampletank free (Custom Shop) you get a grand piano, violins, flute, pop brass, electric pianos, tonewheel organ, acoustic & electric guitars, a P-Bass and a J-bass, and some synth sounds.  I think the SampleTank guitars and basses as well as the acoustic instruments, sound very realistic.
http://www.ikmultimedia.com/products/sampletankcs/
 
These are decent sounds to get started with, and both are fully functional.  Start with these and then decide what you are lacking.  The advantage to either of these is that they are both multitimbral, so you can setup multiple instruments in each plugin instance.

I know Bitflipper is right and I've got to someday make the investment for Kontakt. Also, for too many years, I've been contemplating that Vienna Strings high end package. Suddenly, my little studio, next to the railroad track, will sound like the "Game of Thrones" soundtrack. Compression sold separately. 

I have had Kontakt high on my wish list for a few years.  I have the Kontakt Player, and like the user interface, as well as some of the free factory sounds.
 
But I encountered a crazy deal where I got the full SampleTank 3 for less than <$100, so I probably have all of the sampled real instruments that I need for now.  The only downside now is that I can't spend hundreds more on expansions for Kontakt. 
 
I recently found out that with the Kontakt Player and a free 3rd party library that I had downloaded,  I was eligible for a $250 crossgrade to the full Kontakt.  But I think I will have to wait, because the full Kontakt factory library doesn't seem as complete as SampleTank.  Plus there is quite a bit of overlap with SampleTank.  The advantage for Kontakt is all of the external libraries, both free and paid, that Kontakt provides access to.  The thing you honestly need to determine for yourself, is do you need that?