This is a somewhat subjective question, but I'll try to give you an answer in an objective way. First things first, a disclaimer: I'm part of the Dancer development team, so my opinion should of course be considered somewhat biased :)
Catalyst is more widely used than Dancer, and so there's more community support behind it - if you were to look for contractors with experience working with either framework, say, you'd be more likely to find developers who've used Catalyst. So, if you're looking for commercial support, that would be a good reason to choose Catalyst.
Dancer is a younger project, and targeted more towards smaller projects, making getting up and running quick and easy, and trying to stay out of your way. That's not to say that Dancer isn't suitable for larger projects, however; the same habit of staying out of your way means you can organise your project in the way that suits you.
However, it has picked up a lot of support, and there's a growing community of helpful users and developers on IRC and the mailing list, and more and more useful plugins being released all the time. As with Catalyst, Dancer is designed so that you can pick and choose your preferred template engine, session storage backend etc, and it's easy to extend the framework by writing your own plugins if you need to.
For user testimonials to see what people say about Dancer, see the section at the bottom of the homepage on the new website: http://www.perldancer.org/
In the interests of showing other options, there's also Mojolicious, another modern Perl web framework which has been gaining in popularity lately.