This is an impossible question to ask. Everyone is going to simply recommend what they use. We're a Rails shop. I love Rails. I'd recommend it to anyone, but I don't think someone else is "wrong" for using Pylons/Struts/MVC.NET/etc.
Here's what I've observed about programming languages and frameworks. Each programming language has a community. The priorities of that community will dictate the direction of the framework, and to a lesser extent, the language. Frameworks seem to advance/change faster than the core language itself.
So, rather than asking what language/framework you should use, you should be considering what type of community you think fits your team/product best.
As I mentioned, we love Rails. We've been extremely happy with it, but it's a fast moving framework that requires constant learning and comes with it's fair share of drama. I'm ok with that, because our team doesn't mind the pace, and we enjoy the benefits of a stack that has strong "default opinions". I've introduced several PHP programmers to Rails, and they're constantly amazed at the broad nature of the Rails development stack. Aspects like deployment strategies with Capistrano, and now asset pipelining, are all concepts that the Rails community embrace and support. However, if you're not down with having someone else dictate these aspects of your project, Rails probably isn't a good fit for you. That doesn't make either decision wrong.
If you have an MVP, take your time. Short-list three or four languages/frameworks and do the following:
* Run through the "hello world" tutorials for each.
* Have a look at the documentation for each. How well does the documentation read.
* Pick a component of each framework, like the ORM, and dig in to the documentation a bit. How detailed is it? Documentation that looks good at first may turn out to be not-so-great when you get in to the details.
* Drop by the IRC channel for each. What kind of discussion is happening? How are new members treated?
* Look for mailing lists related to the language/framework. What types of problems are being solved? What's the focus of the list?
At the end of the day, you're the only one who can tell you what is the right fit for your organization. Hell, it might be PHP. Don't fall in to the trap of believing that the right language/framework is going to make your product a winner. It's the people, not the tools, that make the product go.