It depends on the look you are going for. Old school muscle cars all had small wheels with big tires. Modern cars all have huge wheels (3D Carbon went so far to put 23s on their car if I am not mistaken).
I personally think the 18s look too stock and further emphasize the large mass of the car. It will be 20s for me.
But, if you go with large wheels, you NEED to lower the car, or it will look like crap. Also, don't worry too much about getting the same OD of the OEM tires, but get a good sidewall (preferably 40 series tires). The tires will be larger OD, so it will affect your speedo/mileage. Nothing to worry about, just be aware.
The thinner sidewall and lower ride will affect both handling and ride. So, if performance is your game, go lightweight 18s.
P.S. I am 90% sure the BFG ad was photochopped (front calipers are in the wrong location and lighting is a little off), so don't use that as an example. The Saleen's have 20's though, and they look good. They use 275/35ZR20 front and 275/40ZR20 rear tires. Plenty of sidewall, and they fill up the wheel wells.