Are you looking at differences in term of size or strength?
The large bearing is off course larger
But it is more popular with the road race crew who put a lot of lateral stress on the bearing throwing their cars around a corner at 85 MPH and then hammering the throttle 3/4 of the way through to drift out and up to the wall to pass some honda.
If you are looking at installing a 9 inch set up in primarily a street car with a nicely warmed up motor with the occasional trip to the drags or open track (occasional defined as like 1 -2 times a year) you don't need to worry about the large bearing carriers in my opinion. I have a 408 clevaland high horsepower stroker going into a small bearing 57 ford 9 inch housing. For what I use the car for...highway cruising and 1 -2 track trips per year, it is more than enough and was priced quite nice.
Thanks for the help but, I have not seen the actual deminsions. Just to clarify i'm talking about the axle bearings everywhere I have looked it is always listed as large or small. It may just be me but if they don't say large= x.xxx" how am I supposed to know whether I have the large or the small axle bearings?
One advantage of large bearings over small is that you can run axles bigger than 31splines.
As far as knowing which one you have...
Is your axle original? Large bearing 9-inch axle housings were NEVER used on Mustangs. They were reserved in those years to Broncos and F-series. If it is not the original axle or you have no idea where it came from you'll have to measure. The flanges are clearly different between the two, crawl under there and measure.