I just used
blackfire on my Volkswagen GTI and very impressed with the results. Very reflective and the car feels very slick n slippery. As far as ease....went right on and came right off.
As far as durability, Blackfire will last longer.
Souveran is a Carnauba wax, where it may give you a slightly better shine, the durablity is far less then that of a polymer (BF)
Depending where you live, lets say California where the sun is always shining, go the Carnauba route if you don't mind waxing more often because of the warm weather year round. If you live in crappy New Jersey like I do, where the temperature sits in the mid 30s and is too cold to wax in winter (most of the time), and where it snows an inch and they drop 10 feet of roadsalt down, then I would go the Polymer route because you won't be able to clean it as often.