Fifth Gear is made for hard core car enthusiasts who want to be informed. Top Gear is made for the masses of average people who like cars and want to be entertained. From experience, it is easier to get a woman who doesn't enjoy cars to watch an entire episode of Top Gear than Fifth Gear.
Fifth Gear is about the cars first, the location and the hosts come second. Top Gear is more about the guys, the locations and the post production special effects than the cars IMHO.
Fifth Gear's hosts; Tiff, Vicky, Jason and Ben are all professional racers that take that experience to the show. They are professional drivers first, TV Hosts second. Tiff and Vicky are also ex-Top Gear Hosts and worked with Jeremy Clarkson until the troubles that plagued the 2001 season of TG.
It was in 2002 that the current Top Gear format and the then new Fifth Gear both appeared on British TV for the first time.
Now, I like both shows, but Fifth gear appeals to the Car enthusiast in me and pumps more adrenaline in my veins than Top Gear ever has...