I consider them two very different cars. The Enzo is a technical wonder, designed to go as fast as possible using all the mechanical and electronic driving aids available (dual clutch gearbox, traction control...etc). Whereas the Mclaren F1 is built to be a driver's car, with no assists (there isn't even ABS as far as I know). The driver sits front and center for the best view, puts the power down through a proper manual transmission and the rear wheels do whatever your right foot tells them too.
So the Enzo may be faster... but I enjoy my time more in the F1.
P.S. Of course, all the real life specs when it comes to driving aids are all but irrelevant in GT5... but I still feel like the F1 is the more entertaining drive.