For what it's worth, facing a client-side project of similar magnitude, we chose Google Web Toolkit and learned Java.
Two years and 60k LOC later, (www.activityinfo.org) I'm not entirely satisfied with the tool -- sometimes more abstraction introduces new problems to be solved, and it's hard to find good, affordable java devs to work on the project here in NL, but it does IMHO bring tremendous advantages in terms of modularity, dependency management, and generally keeping the code base maintainable. And the optimizing compiler is nothing short of amazing.
I realize the idea of Java-to-Javascript is sacrilege for many, but if you've looking at building a large, single-page javascript application, I think it's worth taking a look at.
Having worked in a project where GWT was used it really makes it easier to work with.
I found it really easy to pick up and start doing some actual work, the only complaint I have so far was the need of everytime I wanted to expose a service from the server to call in the client I needed to edit around 4/5 files, but I'm not sure if it is really needed or if at least 2 of them could be avoided, since there was already a small codebase to follow.
I've been seriously considering ClojureScript (which is on top of Google Closure) for exactly those reasons. The extra toolchain complexity is still an issue though.
Google Closure (JS optimizing compiler and library) has nothing to do with the Google Web Toolkit which compiles Java to JS (GWT). ClojureScript is awesome in any case though :)
Two years and 60k LOC later, (www.activityinfo.org) I'm not entirely satisfied with the tool -- sometimes more abstraction introduces new problems to be solved, and it's hard to find good, affordable java devs to work on the project here in NL, but it does IMHO bring tremendous advantages in terms of modularity, dependency management, and generally keeping the code base maintainable. And the optimizing compiler is nothing short of amazing.
I realize the idea of Java-to-Javascript is sacrilege for many, but if you've looking at building a large, single-page javascript application, I think it's worth taking a look at.