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.