Top Gear all the way.
There is something about the formula of Top Gear lately. When they had the fat bloke during the first few seasons it was a nightmare. When Jeremy, James, and Richard began acting more like friends just having fun with cars and joking around it added a level of informality. Its the formal and scripted reviews that make most review shows awkward.
Over in America we have some really really really bad review shows that literally just run through the numbers and show the car wobbling down a drag strip, then call it a good car. Fifth gear comes close to that, its more of a "this car has ____,_____,____ features that make it a good value."
On the other hand Top Gear is more entertainment. Not very many people are going to even see a Bugatti Veyron in person, much less drive or own one. The fun of watching the show and these insane cars is much more appealing than finding out about what you'll be able to afford and hate to drive.
I equate the success and shortcomings of the two shows to being a kid with a choice of two posters on their wall. The first being a Ferrari racing down a runway, and the second poster being a euro-box sitting in front of a cafe' with two people reading off the teleprompter.
I don't see much of anyone choosing the latter. Hell, I'd rather watch a review of a euro-box on Top Gear and hear how terrible it is, than watch a review of a Aston Martin on Fifth Gear and hear a list of the car's accoutrements.