Oh I totally agree, it is splitting hairs. Like I said, on the street I don't think the difference is noticeable at all. I certainly didn't notice it. For a full on autoX or track car maybe. If I had 73-76 A-body spindles on the car already I wouldn't go looking for FMJ spindles to swap them out. Now, if you're doing a disk conversion using the 73+ disks then I could see looking for FMJ spindles instead because they're usually cheaper and easier to get ahold of. I wouldn't turn down either kind if I was looking for a set of 73+ spindles though.
I think the most important thing is that Ehrenberger's "disk-o-tech" article that talks about "ball joint overangling" and catastrophic geometry issues with the FMJ's is total BS. I wonder how many people missed an opportunity for a cheap 73+ disk upgrade because of that piss poor speculation on his part.
The article isn't clear. I suspect it's total weight for the pair, they're not really that different side by side. I haven't weighed a set before to compare, the FMJ's feel lighter to me but it's not a night and day kinda difference.
It is unsprung weight, so a couple pounds here and there can make a difference. On a street car it doesn't really matter. For a competition car, every little advantage adds up.
To me the FMJ spindles look better. It's just an aesthetic thing. The 73-76 A-body spindles look a little clunkier, like the production guys ran with the first casting mold they came up with cause it "worked the way it was" and it was cheaper than taking the time to clean it up- "just a spindle, who cares". The FMJ's look like someone took the extra time to make sure casting form really matched the nice sexy design drawing- smoothed out the corners, filleted out the excess metal, put a little effort into the details. Both work just fine.