Not sure how we got from Firm Feel to XV, but you get a three way for the price of two!
I think they both compare well. Hotchkis has certainly done their homework on the geometry to improve over stock and their kit may well be technically superior for those that want to X their cars, or otherwise race them (not drag race)
Firm Feel mounts their rear bar similar to the factory which is good for unsprung weight. On the other hand, Hotchkis uses tubular bars which are nice and lightweight plus the rear is adjustable so one can fine tune the handling.
The Hotchkis front upper is a trick piece to fix bump steer which is nice if you are looking for the optimum handling but it is damned expensive. The rear springs are flatter and the eye is rolled differently to provide better rear geometry and a lower ride height.
My gut, unscientific/unproven opinion is that Hotchkis will take you farther but the average guy on the street will probably not know the difference.
If you go Hotchkis, either buy thru one of their distributors or wait for their sales as this makes the price much better. I bought from Matts Classic Chevelle on ebay after seeing it mentioned here. I liked the total package and the ride stance of the Hotchkis package and I bought most of the stuff outside of the front uppers. I thought the prices were comparable to any mixing and matching I tried to put together elsewhere.
I bought some miscellaneous things and the shocks straight from Hotchkis.
Both my dealings with Firm Feel and Hotchkis have left me impressed with their customer service and attention. I have a steering box rebuild and some other parts from FF. They have been very helpful with advice when I asked.