tipoc:
Good question.
If your foe has a light and you don't then he may have a distinct advantage. However, his light may also be a liability if he does not employ it correctly.
Unlike the typical box shaped range, the real world is 360 degrees and that in itself tends to complicate things greatly.
As far as which one would be easier to fight... the guy with a handheld light or a guy with the rail mounted light... that would really depend on the actual situation and how he is using his light.
If you are able to set up an ambush that brings him through a bottle neck (like a doorway), then it likely won't matter what he is using... you'll have the drop on him.
On the other hand, if we are in a face to face engagement... we are likely in some very deep poo... especially if he knows how to use his gear.
For a head on situation, we may try for a shot below the light (to cover the rail mounted technique and the over hand or Rogers/Surefire technique) and another just to the left (to cover a right handed shooter holding his light out to the side in his left hand). Who knows, we might get lucky.
A few principles we can put to good use in a low light situation...
Stay in the dark -- Move out of the light and into dark spots
Watch your backlighting -- Don't stop in doorways and never allow yourself to be backlit (the #1 mistake)
Light and move -- Use brief flashes to search dark spots or find your way to a new location
Randomize -- Strobe the light on/off and move it randomly... high/low and side-to-side of your centerline
Of course there is no substitute for good training.