Ok -
I don't really care about this - I have the 50 1.4 and the AI-S version, I have the 50 1.8 too.
I am a big fan of the M/A switch so that you, mid shooting down have to take your eye from the camera and switch - that's a baby issue. I like the manual focus over ride which makes a world of difference on the higher up lenses. I use it all the time on my 70-200 especially for concerts and things where I trust my eye over the AF system. AF-S is considerably better than the D versions. That split second of tracking can make or break a picture.
That being said, those are all little things that don't make up for the price difference in my opinion. But how bout this, buy a - now - second generation 50 1.8 for less and fucking shoot with it and shut up :)
Look, you can argue canon you can argue nikon. I chose Nikon because I already had a substantial amount of money invested in lenses. They both take good pictures, and if you're blaming gear or a company for the inability for you to take better pictures at the nikon and canon level then you're stupid.
Hell, Pentax, Olympus and other cameras take great pictures too. I don't see people whining about them. I know a National Geographic photographer that is sponsored by Olympus and his pictures are sure as fuck better than 99.99% of everyone else's pictures.
Sure, canon lenses are cheaper in some regards but deal with it. I would stick with Nikon if I still had my choice today - I have lenses that Canon doesn't offer and the ones that I do I like better than Canon's.
At the most basic level, gear doesn't limit your photography - it's you. And I'd say that lenses make up 75% of a picture the camera a bit and the user all of it.