You already have a 50mm lens, right? The f/1.8 version? That's a fine lens for now. A longer focal length lens, like the 100/2 or the 135/2 referenced above, will give you a tighter angle of view for portraits and candid action photography.
The 50/1.4 or the 50/1.2 have a larger maximum aperture, so they let in more light, at an increase in weight and cost.
Since you already have a nice wide angle and "normal" lens (the 28 and 50), the next logical lens would be an 85, 100, or 135. In my personal opinion, the 100/2 is a good choice. All that said, if you are willing to try a zoom, and have some extra cash, the 70-200/4 or 70-200/2.8 really are worth every penny to a professional wedding photographer. The f/2.8 is the better choice for this, but a lot more expensive. (Makes the 100/2 look like a bargain :-)
I always let my assistants and students borrow gear. Perhaps the photographers you are assisting can talk with you about your choices.