Not much difference, but the FJ springs seem to be more "progressive" meaning they are smoother on typical everyday bumps.
The different shocks make a lot of differenct as well. For example, Bilistein makes the 5100 series for all Toyota's built on the 120 platform which includes the Tacoma, FJ, and 4Runner.
I have put FJ springs and Bilistein shocks on my 4runner and it is MUCH smoother riding. My wife could even tell the difference. Not sure why they didn't use the same springs to begin with, but they should have.
^ + 1. FJ springs smoothen out the ride because they are a tad bit more stiff, and then the shocks (especially the FJ Bilsteins) seems to work just as good if not better than the OEM 4runner (both standard and XREAS). 12guns setup is awesome, I was amazed at the difference in ride quality with his vs mine (I have just front FJ springs, stock XREAS). Only thing is he went with the FJ coils and 5100's instead of the FJ shocks.
The FJ shocks were appealing back when the FJ owners were reasonable on them, but now there are better aftermarket options (like the 5100's). Guys are asking $250-$300 for a used stock suspension, you're much better off buying the 5100's and trying to get a deal on some springs. You may get lucky and find a better deal on shocks/coils, but prices have skyrocketed with the demand from 4runners (which is understandable, but I can't seem them asking the same as what new shocks go for).