<p>Up till now all my programming experience has been desktop development (mostly C/C++ with OpenGL/DirectX) but I'm interested in trying my hand at some web dev.</p>

<p>The two directions I'm considering are Ruby on Rails and ASP.net.  </p>

<p>Which is most widely used?</p>

<p>Which would be a more marketable skill to have?</p>

<p>Thanks!</p>

Learning Ruby will help you learn a different way of thinking about programming. It will help you become a better programmer, even if you determine that it's not a language for you. The same applies to several other languages (including Python). There are fewer job opportunities for Ruby developers.
ASP.Net is a fairly pedestrian way to develop fairly pedestrian webapps that don't really challenge how you think as a programmer. Job and consulting opportunities are more common.
From this point, it's your call. I chose Ruby six years ago.