Tim Bray compares Frameworks (Source: Still done seeking). I largely agree with Tim Bray's analysis, particularly the point that all major web frameworks will scale sufficiently and that, in the long run, maintainability is likely to be the most important factor in the attractiveness of a framework.

 By that token, Bray recommends Rails and Java, and is skeptical of PHP.  I think this is probably right, and that ASP.net, using C# or Visual Basic, would compare to Java.  The Rails advantage, according to Bray, derives from Ruby's increased expressiveness: fewer lines of code means clearer code.  While that's often true, it ain't necessarily so (metaprogramming can be very hard to understand). Further, implicitly typing language has disadvantages for maintainability.  Further, one should not ignore the ease with which one can hire a maintenance programmer, which will be certainly easier if one chooses a Java or C# framework.

In my opinion, the jury is still out on the relative maintainability of Ruby on Rails versus Java/Struts and ASP.NET.