Having used both, I would say that it depends on what you look for. In my opinion:
Chef is more developer-oriented. If you're a Ruby guru, you'll love it.
Puppet is more sysadmin-oriented. It has a non-ruby DSL so it's more difficult to propagate mistakes to your machines (imho).
Puppet creates more readable and stable code but it's also slow to deploy new features. That's probably what you'll want in a big enterprise structure which strongly believes in your DevOps work.
With Chef you can achieve complex tasks with less code, time effort. You can use all the ruby magic without having to create a Puppet construct. This is good, for instance, when your Company doesn't truly believes in DevOps value and you're constantly struggling against time to prove your manager wrong :-) I personally find Puppet a bit slower to execute when you develop new features, which can be a bit of a pain.
My suggestion is: if you're a sysadmin with some development skills, go for Puppet. If you're good with Ruby (or Python), go for Chef.
I also tried rump and I'm playing with it. It helps, it's cool, but I still don't see a huge value except lazy typing of rump go instead of puppet apply -vd --modulepath=. module/manifests/init.pp. :)
