The real advantage to a 9 inch rear end, other than their phenomenal strength, is the ability to set up a few different gear ratios, in different center sections, so that the gear ratio is correct for the track conditions. The center section is relatively easy to remove and exchange with another center section with a different gear ratio already installed. It's not really necessary in street or drag racing cars but it can be beneficial on road course cars and especially on both dirt and paved oval race cars, to be able to quickly adjust the gear ratio at the track.
A Ford 9 inch is a sort of "Poor man's" quick-change rear end.
I don't know of anyone on this website that has a car that is putting out enough power where they would have to upgrade to a 9 inch. The 8.8 is very strong. Ford replaced the 9 inch rear end with the 8.8 because it is less expensive to manufacture the one-piece rear end as opposed to the two-piece 9 inch. The 8.8 is used in all of the applications that the 9 inch was used in and it performs quite well.
A nodular case 9 inch is undeniably stronger than a 8.8, and the versatility is great for some applications, but I would just stick with the "beefed-up" 31 spline 8.8 until I, literally, grenaded at least a couple of differentials.