First, I suggest you read my post on the subject:
http://blogs.microsoft.co.il/blogs/idof/archive/2012/03/05/wcf-or-asp-net-web-apis-my-two-cents-on-the-subject.aspx
Regarding your WSDL question – since the WebApi does not use SOAP, it does not require a WSDL, and does not export one. You can use Hypermedia to return resources with a list of possible activity URLs (think of it as a self-describing resource)