With zero-yaw flow,for a leading edge,your after just enough radius to get the flow attached,which would be a fraction of the body width,or a fraction of the square-root of the frontal area.It's been researched both ways.
If you do the blown slot as shown,the upper surface is above the plane of the roof,which is separation right there.
The slot introduces a jet of air which disrupts the pressure profile of the air around it,adding an aerodynamic 'kink',to the body,and it aggravates the growth of the local boundary layer,which we'd like to minimize as much as possible,as well as have as uniform as possible.
For crosswind conditions (mostly the real world of motoring) a bulbous nose has an advantage over all other leading edges.
Here's some examples of 'fixes' for a trailer
Killing the gap is even better
