The Mark II has a more aggressive, powerful and intimidating appearance, what with the extra scoops. It was also the most powerful on the track (excepting the Mark IV, which was an entirely different car) with the unprecedented 1-2-3 finish in '66. It will be the one I will get (if I get one at all).
But as for NSXER, the Mark I's have their appeal. It all depends on yer druthers . . . . . .
By the way, the Number 2 in the image above, if it's #1046 and not a copy, is the car that won in '66.