The information below is my personal opinion based many years of night driving with a variety of different lights and over 25 years of auto electrical knowledge. Please note the following article assumes only one pair of driving lights is to be installed.
Bigger is better right? So if the manufacturer claims their light shines two kilometres down the road, then it must be heaps better than one that only shines seven hundred metres. Here are four reasons why I don’t think so! Maybe the two kilometre beam might be useful as a shooting spotlight with a high intensity spot of light right where you aim it, but on the front of you car, can you move that dot of light to where it needs to be? I have driven many a vehicle fitted with two pencil (spot) beam driving lights shining two skinny beams of light way down the road. Firstly, can you see a kangaroo on the road two kilometres away in daylight? So how are you going to see it at night? Secondly the intense pencil beams create a bright spot in the distance while all around is dark, causing your eyes to focus on this one patch of light. Thirdly, when you are cornering the two spots of light are both out in the bush ahead of your car and there is no light where you’re actually about to drive. And last of all, pencil beams are very sensitive to adjustment. A little to the left or right and they are no longer shining down the road at all and if you put a load in the back then you will be looking for owls in a tree two kilometres away.
Most commonly driving lights are sold in combination kits with one spot and one spread beam. This provides the best of both worlds for those that believe they need light over a kilometre away. When mounting these lights the spot should be mounted on the right hand side of the vehicle so it has the best chance of shining up the middle of the road. (A mistake you see quite often)
My preference is a pair of dedicated spread beams (not adjustable, more on that below) ideally with free form reflectors. (see Narva Ultima 225 or Hella Rallye 4000 Compact for examples) Properly designed spread beams provide a uniform spread of light without any bright patches and will illuminate the road from verge to verge and up the road to cover your braking distance. Slight misalignment won’t cause too much concern and you can turn them on a lot sooner or off a lot later when following another vehicle if you’re not giving them high intensity flashes of light at two kilometres.
Free form reflectors are the modern way of shaping the beam, using the reflector rather than the lens. This eliminates the need to use glass for the lens resulting in a more transparent and stronger product.
Why not adjustable? Several manufacturers offer driving lights that can be adjusted from spot to spread. (Like many torches do) This is done by shifting the focal point of the light source (globe) in the reflector. Firstly by shifting to a spread beam you are simply creating a much bigger circle of light, rather than a shaped spread beam. Much of the light produced is now shining as a big arc up in the sky and some more of it is now right in front of your vehicle, too close to be of any use when you account for reaction times etc. All this wasted light is refocussed into the shaped beam in a dedicated spread beam light. Secondly reflectors are a parabolic shape and have a focal point, dedicated spread beams keep the globe in the focal point intensifying the light output, rather than shifting it out of the focal point as adjustable beams do.
Round or rectangle lights? A reflector as mentioned above is a parabola and a free formed reflector has some angled surfaces within that parabola to alter the shape of the beam. A rectangle light has the bottom and top of the reflector flattened out, so therefore results in an area without a proper focal point and some light that really never shines out. I have never seen a test or comparison of lights where a rectangle light has come out on the podium. In saying that, if looks or space dictate the shape then as a spread beam they are not too bad.