Interesting stuff...

Thanks!

I generally use VST3, if available. Some programs have had issues with it, I think IK did for awhile, but in general when companies do VST3 that is what the focus on. Also it does offer some features that VST2 doesn't. Not all plugins make use of it, but there you go.

u-he synths have had a known bug with VST3, unless it's been fixed already, that prevents the VST3 plugin from recalling the saved preset with a project. The result with the VST3 version is that the plugin opens up with the default preset, even if you have saved the project with something else.
 
Their recommendation has been to stick with their VST2 versions until they resolve the compatibility issues with VST3.  They stated that there is no other functional difference in their plugins using VST2 or VST3, so no compelling reason to migrate to the new version.

I'm still using vst2 for TH3 full version because vst3 doesn't respond to program changes from my midi footswitch.  As far as I know that's still the case although I haven't tested recent versions.  From a previous post, this is what Overloud said about it around June 2016:
 
Yes, this is an issue with the VST3 format and we are currently working on it. 
If you wish to use the latest version of the plug-in and correctly receive Program Changes, you should use the VST2 format.

Ran across a couple of other posts regarding this topic:
 
Should I use Only VST3s Now?
https://www.bluecataudio.com/Blog/new-releases/vst3-hosting-released/
 
VST or VST3
http://forum.cockos.com/showthread.php?t=181780
 
I'd say that VST3 has some advances and new features that look great, but keeping the VST2 around for compatibility is not a bad idea.