I would go with Selenium. I say this for one main thing: open source.
I cannot begin to explain in words, how annoying it was to be calling something from Selenium and it was doing something I wasn't expecting or I'd pass in a value and it wasn't doing anything. Documentation can be a little patchy, so whats next? Look at the source code.
I've learnt how Selenium works back to front by looking at the source code. With Telerik's, you obviously cannot do this.
With browser support, they are the same.
Both are well written, well maintained, Telerik's has dedicated support whereas with Selenium you are stuck to Google groups, Google and Stackoverflow.