Do I need Ruby or not?
There are two versions of ConTeXt in use, MkII and MkIV. MkII uses
Ruby and pdfTeX (or XeTeX) and is the frozen stable version. MkIV does not
need Ruby, it is launched using
context and uses LuaTeX as backend. This is
the current development version.
If you don't have a good reason to use MkII I would advice to go for MkIV and
not care about MkII.
Does that depend on the installer, current tex distribution, anything else?
If Ruby is installed, by default both versions, MkII and MkIV are installed. If
Ruby is not present only MkIV can be used.
Does the (one of the) GUI installers work? What is the difference between that and the command line installer?
I never used a GUI installer. I would suggest to just try if it works. It
should install the same distribution, ConTeXt standalone. In any case the
command line install (using
first-setup) is the preferred and supported
method which should work in any case. You'll get a better experience if you
also install the optional helper
If I install alongside a working tex distribution, do I want the environment variables set globally or not? Why? Why not?
This depends on your workflow and personal preference and how you launch TeX.
If you prefer to launch it from the command line or from an editor. If you
mainly use MikTeX, you might want to have MikTeX in you PATH and not ConTeXt
standalone. I personally have the ConTeXt standalone beta in my PATH by
default. If I use older versions which are local to a project, I set up the
PATH locally for that shell or within that editor session.
Do I use texexec.exe/rb? Do I use context.exe?
texexec launches MkII,
context launches MkIV. As described above, you
probalbly don't need to care about
texexec. In the older documentation the
texexec can be replaced by