Another way to look at it is how many Lumens (the unit of light measurement) does your headlight provide? For example, a 6 volt incandescent bulb at 6 amps can produce the same amount of Lumens as a 12 volt incandescent bulb at 3 amps.
The weakness of the 6 volt system is that it is more susceptible to resistances. Battery post, starter post, ammeter in, ammeter out, headlight switch in, headlight switch out, inner fender terminal strip, headlight socket contacts and the wiring itself all are a source of resistance. At 6 amps, these resistances cause a voltage drop that impacts the available headlight voltage more on a 6 volt system than the 3 amps of a 12 volt system. When our trucks were new, there was no problem. Time, moisture, dirt/dust and corrosion is a lot of the reason for the complaints and comments of today. Elbow (cleaning) and dielectric are the greases that I consider the low cost solution. Switching over to a 12 volt system, keeping the original components, the resistances are still there but they have less impact. Most conversions to a 12 volt system include a complete rewiring so most of the resistance issues have been removed.
In going from incandescent headlights to halogen headlights, the bulb current is about the same (I have not verified this). Halogen is brighter to the eyes. The plus side of Halogen is that the bulb lasts longer.
