Side-note #1: The terms front-end and back-end is not necessarily the same thing as client-side and server-side. But for this post, this is how I am using them.
My guess: very much more productive.
Side-note #2: Sometimes additional, more special-purpose languages are a good thing. There is even an acronym to represent the concept: DSL - (Domain Specific Language). Examples of DSLs include things like HTML, CSS, SQL, XSLT and regular expressions. But when I speak of a single programming language for front-end and back-end, I am talking about general purpose languages. The predominant front-end and back-end languages mentioned above are general purpose programming languages.
So, if you would like a single end-to-end language for web development. Here are your options:
[Update: 7/10/2012] : This post pre-dates nodejs and dart, both of which enable same-language development on client and server.
ASP.NET and JScript
My Own ExperienceI have dabbled with all of these technologies. But the ones I have the most experience with are GWT and JSF/ICEFaces. Presently, my favorite tool for web development is GWT. It doesn't have the most feature-rich widget library in the world. But that is because they focus their efforts on making the "core" as good as it can be, then leave some of the widgets to third party vendors (of which there are dozens). If you work with GWT you will become a better web developer. There is some serious "smarts" contained in GWT. And it will rub off on you. Things that took days in JSF take minutes in GWT (creating your own component, for example). GWT is a thin layer on top of DHTML. It doesn't try to hide anything and generally doesn't create a whole new abstraction layer. But I could go on all day about GWT, but that would be a subject for a different post. Here are a few links about the general philosophies behind GWT:
GWT Mission Statement
GWT Design Axioms