The build quality of the 50L alone is worth it for me. The 50 1.4 is plastic-not-so-fantastic. IMHO.
I'm sorry, but this is the weakest possible argument for spending all that extra money, unless you've had a 1.4 mechanically fail on you. Build quality adds nothing to the photo.
On thinking further, I guess you're after more than a good photo - a better photographic experience, i.e. the pleasure of using quality tools. If that makes your hobby more enjoyable then good for you. The price difference must mean less to you than it does to me.