What is better for lat development, lat-machine pulldowns or chins?
Without a doubt, chins. Many bodybuilders feel that pulldowns are as effective for developing the lats as chins, but that is just not true. Chins are the better developer because they are harder to do. When you do pulldowns your body and torso are stabilized and you pull the bar to the chest or upper back. When you do chins you have to pull your body to the bar, a much more difficult task, That's why free-weight exercises are more difficult than exercise machines - and better muscle-builders. You are forced to work harder to perform the exercise and stabilize the body at the same time.
In the same way, leg presses are a very good leg exercise. But leg presses are ultimately not as effective as squats with a barbell. In order to squat with heavy weights for high reps you need strong hips, spinal erectors, glutes, hamstrings, quadriceps and calves. You can leg press pretty hefty poundages with a relatively weak back and hips, but you will never get the maximum growth effect you'd get from squatting.
Pulldowns are a good secondary exercise, excellent if coming back from an injury or training around an injury, but if you want a big, wide back, perform chins. When you are strong enough to perform several sets of wide-grip chins for 10 reps with a 50-pound dumbbell hanging from your waist, your lats will be about as wide as you'll want 'em. Your biceps, forearms and shoulder width will be greatly improved too. When you do wide-grip chins you "spread" the scapulae and help stretch the clavicles in a way that pulldowns can never accomplish. You have to chin if you want to really get impressive V-taper.