It depends on the exercise.
With lateral raise its best to use a dumbbell...
Because your moving the weight from a low possition (where there is little stress on your delts) out to arms length (where the muscle is working very hard to keep it up)
the weight, in effect, gets heavyer on the way up
with a cable, the resistance on the delt is most at the bottom. When your at the top part of the motion the chest and triceps are working to hold the weight away from your body (pushing)
So the weight gets lighter on the way up.
However I love the cables when it comes to dule cable flyes.
With the cable you can bring your hands all the way and touch them out in front of the body. If you did this with dumbells on a bench you would loose the resistance on the pecs and just be holding them up with your triceps.
So... when it comes to flyes you get a better range of movement with a constant resistance, but with lateral raises you loose resistance through the motion.
P.S. oh yeah, that was my example of positive and negative arguments for cable machines over dumbbells for my gym instructing course.