Hello.  What is the difference between VST and VST3? 
 
Some of my plug-ins (Z3TA+ and Rapture Sessions) appear in my Plugin Manager in both VST and VST3 versions.  Which version should I use?  Is there an advantage to using the VST3 version over the VST version?  (I am thinking that once I figure out which version I will use, I will hide the other version.)   
 
Thanks.

Hi. I seem to remember reading that VST3 is less resource hungry. And has better coding, if I understand correctly. But that is where my (lack of) knowledge ends. Other will know better.
All the best.

Ps. I use VST3 exclusively instead of the VST2 version. You can tell SONAR to exchange VST2 for VST3 if available under Preferences (P), Vst Settings. 
 

You can find information on VST3 here:
 
http://www.steinberg.net/en/company/technologies/vst3.html
 
For me, the main 2 points are improved performance, and dynamic allocation of inputs.  No more mono and stereo plugins to mix up.  VST3 determines whether the source is mono or stereo.

Hello.  Thanks Dave and Zargg for your helpful replies.  So I will do a little reading on VST3.

The other difference is that VST3 will eventually replace VST2, so that's where you're going to want to invest. VST2 is on its way out. However, at this somewhat early stage, not every DAW is VST3-ready, and some that are still run into issues with VST3 plugins because the plugin developers are still learning. It is good to have both, for now, so if the VST3 doesn't work correctly, the VST2 may.
 
Regards,
Dan

Some people have already started having problems with the Sampletank 3 VST 3 plug, or so I have heard. I know mine was kind of buggy so I switched over to the vst 2 version and never had a problem.

VST3 plugins are not more efficient. In all likelihood you will experience exactly the same performance with either VST2 or VST3. Developers maintain a common code base for both, with 99.9% of the internal code being identical. The idea that VST3 is more efficient stems from its feature for shutting off a plugin when there is no data coming in. However, plugins have always had that ability if the developer chose to implement it. 
 
VST2 is not going to stop working due to obsolescence. Some new products, in particular those from Steinberg/Yamaha, will only be available as VST3. But as long as the host supports the VST2 standard, your VST2 plugins will continue to work. Eventually, vendors may decide to stop developing new VST2 plugins just to simplify things for themselves. Eventually, VST3 may be the only choice for new products. But even then, your existing VST2 plugins will still work.
 
To answer the original question (what is the difference between VST and VST3?), they are two versions of a written specification describing how hosts (DAWs) and plugins (VSTs) communicate with one another. That's it. It does not dictate how plugins work internally.
 
VST3 does add some new capabilities, mostly relevant to soft synths, such as support for multiple MIDI ports that can be switched on the fly. There is a myth that only VST3 plugins can use external sidechains, but that only applies to Cubase users. We've been doing it in SONAR for more than a decade, long before the advent of VST3.
 
Being a new standard, many developers have hit snags while adapting to it. Consequently, many newly-reported bugs turn out to apply only to the VST3 version of a plugin. When that happens, you can simply revert to the VST2 version and carry on.

Importantly, the automation menu in Sonar is much better organized for VST3s. You can find each parameter by category instead of long random lists of 128 parameters. That is, if the synth manufacturer took advantage of the categorization options.

Hello.  Thanks guys, for the interesting and enlightening discussion.