Hi,
You are correct, that there are differences, but still, those differences are small enough that you can revert to MSDN documentation for the majority of our controls to learn about a particular property or method. We try to make those differences minimal, apart from the fact that for Windows Forms you are programming for desktop while in Visual WebGui you are programming for browsers, so there are bound to be platform and implementation differences.
As for .NETHTML5 versus WinWeb, you really don't have to make any finaly choice, as you can easily switch between the two versions by uninstalling one development environment and install the other, clearing cache, rebuild and you are ready running on the other version.
For production, you need to keep in mind that .NETHTML5 is a Preview version and as such, not recommended for production at all. Having said that, the difference between .NETHTML5 and WinWeb is almost all in the rendering engine, which means that at the moment, those versions are almost identical. Still, there is a different rendering engine, and .NETHTML5 *is* a preview version.
This will always be your own choice, but personally, I would recommend that for production projects, you should stick to the released version.
Finally, licensing is explained on our FAQ page here.
Hope this explains,
Palli