Help needed Choosing a Ruby Framework

I'm helping a friend who is going to be creating a new web app a hand navigating the technology maze. So far I've steered him in the direction of Ruby because it seems that a small team can build a good web app quickly in it. So far so good. I've read a most of the Pick Axe book and it’s great as far as it goes.

But now I'm stuck how do I evaluate the various frameworks? Rails? Merb? I like the agnostic approach of Merb and am concerned that Rails has already chosen the UI library (the ORM tool, etc.) for me. But I know so little at this stage I may be missing the boat. Where do I go to learn more? The ideal would be I write small web app in each framework to get sense of what works. But this is unpaid work and I have a family so that option is right out.

I’ve found a news item at InfoQ: The Forgotten Ruby Web Frameworks - it at least mentions other frameworks. But even that news item makes no real attempt at comparison. Are there any good comparisons of the frameworks out there? Where would you start?

To be the decision doesn't need to be made right away and there will be room for some experimentation once his developer gets going, I'm just trying to set him on a good path.