Based off of what I have learned with light selection, here are some things I would keep in mind when determining what light optics to select.....

1. Are you trying to illuminate an object that will be moving?  Will the mounting location of your lights move up/down?  If so, I would select flood optics.  The challenge with trapezoid optics is that they are very focused, generally to the tune of a 15 degree vertical range and 50 degree horizontal range.  So, if you are trying to illuminate an object that will be moving up/down, using a trapezoid optics can result in rapidly going from 100% to virtually 0% illumination.  Flood optics generally have a 45 degree vertical and 45 degree horizonal range.  This is why you see someone like Deere using flood HID lights to illuminate the boom on their sprayers.  It is possible to have a motorized light that follows a moving object but that is $$$$$$$$$$$$$.   
2. If your target object and/or light mounting location will not move up/down, I believe you will get along well with with flood or trapezoid optics.  The trapezoid optics have the potential to provide a much more concentrated level of illumination, however, there is another challenge.  Light illuminated in a trapezoid beam needs room to "expand."  For example, if you have a 3-point mounted planter and mount a pair of trapezoid HID lights on the fenders of your tractor, you may find that there is not enough distance for the trapezoid beam to "expand" and provide the coverage that you want. 
3. There may be a place for spot HID lights but I have really struggled to determine what it is when taking into account the cost of HID lighting.  Generally, spot HID optics have a range to the tune of 5-10 degrees vertical and 5-10 degrees horizontal.