My guess is the 18's got everything going for it, better tire selection, quicker turning, lighter.
The only thing going for the 19 is it's easier, and maybe cheaper, to find a matching set-up for the 450 forks, ie; hub/axle/fork interface. I still haven't received my CL450 wheel, I hope I'm right about all that, 'cause that's why I went that route. $65 bucks for the complete wheel and brake/hub.