I own both 1.4s and have used the 1.8. Just got the AFS for Xmas and have been testing it endlessly.
Overall here are the main points I have found:
If the AFS is slower I sure can't tell tracking two puppies in a frame. It is able to focus between objects faster and can hunt if you are not using single point AF. That is the only way I can see it being slower. The focus on single point is faster than my old AFD.
Manual override is fantastic to have especially with the shallow DOF of the 1.4.
The new AFS bokeh has rounded highlights where as the older AFD and 1.8 have a hexagon shape. On my D300 I use it as a portraiture lens and the rounded highlights make a difference to me personally.
Big bonus - The front lens is inset a good distance from the front. This helps with flairs and the ability not to use a hood.
I respect KR for showing that cheap or older lenses can have very little trade offs. He does push cheaper alternatives to the best lenses - that is until it comes to Leica, then everything else is garbage. If you look at what he really uses, they all are the highest price or the newest ones for almost all of his images.
If you are not worried about price, the AFS is the better combination of everything. The others are good lenses as well, but this is a great upgrade!