I think they both copy from each other.
Top Gear has probably proven more popular which is why there are a few similarities now being shown on Fifth Gear, but I like the fact they're driver oriented.
Both Tiff and Jason are BTCC champions. They've got infinately more cred as racers and drivers than the TG crew, but I appreciate TG because it's more light entertainment/comedy with a focus on cars, rather than FG, which is a dedicated car show. This is very true also with all the crash testing and more scientific testing they (usually) do.
Both have their place in the market, and I like each for different reasons.