The 24mm f/2.8 Ai is great. Has a wonderful look to it but gets "smeary" in the corners.
The 24mm f/2 Ai or AiS are decent but I think they stretch the design too much. I had a perfect sample of this lens and just didn't like the results. It's got too much coma for my taste. If I'm going to have that much coma in a lens, it ought to be a 35mm f/1.4 or 50mm f/1.2.
The 28mm f/2.8 AiS is fantastic because it is so highly corrected against distortion. It's very sharp if you get a good, clean sample. It excels at close distances and when you put on an extension ring to get closer, it just gets better. Problem is you have to get so close to your subject that you need a ringlight or cross-lighting from strobes and have to watch for shadows.
For anything with straight lines in it, this is a great lens because it has no detectable distortion without loading into a PP program and sticking rulers on it then pixel peeping.
If you have a 40mm already then a 28mm might be too close.
If you want to get really wide, go for one of the 20mm lenses. The f/2.8 AiS is the best overall but I personally prefer the f/3.5 Ai because it's not much more than a pancake lens. Very small, light and a fantastic performer. Not much bigger than a squash ball so it can fit in your bag or even a pocket easily.
Phil Forrest