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.
The two directions I'm considering are Ruby on Rails and ASP.net.
Which is most widely used?
Which would be a more marketable skill to have?
Thanks!
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.
If you are planning to do enterprise business development, then ASP.NET wins hands down (IMHO).
If not, RoR would definitely add to your breadth of knowledge.
PHP, Ruby On Rails, ASP.Net, or Java.
It's a religious choce and it depends on who you ask.
Everyone you ask will give you a different answer.
You should ask yourself how you want to work, PHP java and ASP all let you write markup that is interspersed with code or code that writes the markup for you.
To be honest it's subjective and no one will be able to give you a straight answer.
Given your two options ASP is probably a better choice for industry though, there is a lot of money in it and C# is close enough to C/C++ for it to be readable.
I spend most of my time doing ASP.NET development and recently I had taken a turn against it due to the difficulty in really testing my apps. Like lubos hasko outlined above, with ASP.NET MVC, LINQ and the new features in C# and even the ability to write .NET web service that can be consumed by Silverlight, I have gotten excited about it again and think you could do well getting into it. That said, I have no experience of Ruby but after some of the comment on here, I think I'll have a look at it.
As a C++ you probably won't like the nature and character of Ruby. You might even hate it. Type safety and strictness is an issue of importance for most C/C++ programmers. If you move into a type-free, "dynamically maliable at run time" kind of environment like Ruby, it will feel extremely sloppy.
C# would be the much easier move, from a language and thought-process standpoint. I would not recommend doing classic ASP.NET development anymore. So-called WebForms are quickly becoming outdated and outmoded. The 4th rev of Microsoft's MVC framework just got published a few days ago, and it is pretty terrific. This is the way to go.
So why not just use Ruby on Rails? Why use Microsoft MVC? There are very good reasons why you should.
Why don't you take a few days or a week to experiment with Rails, just for fun? You might, like many other devs before, find a real liking for it and 'fall in love' with Ruby and revitalize your programming interest. If not you just embrace ASP.net which will feel more natural to you anyway.
Other commenters have not mentioned that the number 1 advantage of both Ruby and Rails is 'pleasure of programming'.
Alsa these days talented Ruby programmers are in very high demand. There is much more demand than supply. You can do the math as far as earning potential.
I would suggest taking a look at the Rails for .NET Developers book before making your decision. At the very least, have a trawl through the Softies on Rails blog.
Beware of the "there are more .NET jobs around" argument. That may be true, but in tough times (i.e., now and for the next year or two), you're probably going to see more .NET developers fighting over fewer and fewer jobs. Lots of jobs in good times equates to lots of competition in bad times. So it is worth branching out and learning something new. What is key, in terms of marketability, is staying agile rather than sticking religiously to one technology.