If you want a basic data driven application which isn't going to have a significant load, then I would go with RoR. I have personally found that when ever you go beyond that use case, you will very quickly find the limitations of the framework (e.g. you can't start a sepeartate thread in an action). Yes there are workarounds to these problems, but they are all rather experiemental and a bit hacky.
For all the thats bad about microsoft, the fact that they are making ASP.NET for profit means they are designing it for large companies which have very complex problems to solve. This means that ASP.NET is designed to be highly scalable and efficent.
What I would say is avoid WebForms at all costs, it is a horrible framework designed for desktop app dev's (using WinForms) to quickly get to grips with web development. Unfortunatly to get WebForms to act like a WinForms app requires a lot of horrible things (i'm pretty sure that satan invented ViewState). Also, as xenoterracide mentioned it will run best on Windows Server meaning you will probably have to go down the microsoft route and spend crap loads of money.
A better option would be to look at Castle Project (http://www.castleproject.org/) they have basically developed a port of Rails for C#. It has all the core concepts of Rails, works brilliantly on MonoRail and uses NHibernate at the core of it's ORM and so can you use whatever DB you like.
I think in the end, you shouldn't be looking at the language and saying this ones got more hype I should use that one. I would say look at the actual problem you are trying to solve, what are the requirements? Do you need something out in a couple of days which just does adds a few records to the DB? go with RoR. Are you making a massive data mining app which is gonna be run by 50k people? Go with ASP.NET (or MonoRail)!