The overall radius of 18 19 or 20 is the same so fuel economy is only effected by the wider tyre having more wind resistance and the increase in rolling resistance.

Frazer is correct in that the lower side-wall will transmit more bumps to the car and the wider tyre will cover more road area and 'find' more bumps.

The main problem on tracks and ruts is with the lower side-wall the rim is nearer to the ground and more prone to damage from the side of ruts and rocks etc in the road.

I went for 19s but I have to say if I was spec'ing the car again I would stick with 18s and save £900.