My bad, I should have been more clear. The default 500s are 7k front/3.5k rear, while the default 510s are 4k front /3.5k rear- I wonder if it is meant to dial in more oversteer in the racing application?
I've installed at least 20 sets of Fortune 500's on Subarus and I can tell you that they're fantastic. Hands down the most comfortable daily coil I've driven on. 510's are also fantastic, but purpose built. They're also modular and can be re-valved/built into 2-way and 3-way systems.
TL;DR
90% of people should buy 500s. You'll never be happier with a coilover, I promise.
10% of people should buy 510s and only if they're serious about chasing down tenths. If you just "track" your car on occasion, I wouldn't blow the money. Take the money you save buying the 500s and put it toward an LSD.
Cheers!