2 Stops. You can shoot in a fairly dark bar (or other dark location) with the 50 1.4. My oldest has a band and I usually shoot my D700 with 50 1.4 @ 1/90 sec F 1.4 auto ISO to 6400. In fact my bags packed and we're leaving in about 20 minutes to catch her show. For the price (older D or G) it's a no brainer.
Do not settle for mediocrity. Rather strive for excellence for even in that attempt lies a measure of success.
i have the new 50mm 1.4 and use it on both my d300 and d700. the IQ is outstanding even wide open and the 2 extra stops of light mean i can shoot the kids inside using window light till almost dusk. the other advantage of the extra 2 stops is in very low light shots you can keep the iso down a bit more. i have the 24-70 2.8 on the way but i will still use the 50mm alot.
Yep, 2 stops and more control for DOF, and sharp even wide open. Having said that, I don't use my 50mm f1.4 as much as I did before the 24-70 f2.8 entered my inventory, but I still glad I have it, 2 whole stops is not to sneezed at.
Another factor that is often overlooked is an f1.4 lens will give better autofocus performance in low light situations than an f2.8 lens. Even though you may be exposing at f2.8 or f4 for DOF reasons, the camera will be autofocusing at f1.4.
Not to mention the fact that you don't have that big old thing hanging off the front of your camera. Much easier to handle, a little more obscure (especially without the grip) and you can just move a bit closer or further away with the same result. The 50 1.4 stays on my D700 about 80% of the time.