Pull-ups done properly are far and away a better back builder (width and overall musculature) than lat pull-downs, the only exception being, IMHO, doing lat pull-downs with a narrow V-grip attachment with a heavy weight for multiple (4 - 6) sets of 6-8 reps per set max to blast your inner back and upper traps (along with your bis, of course).
If you can crank out 15 or more wide-grip pull-ups from almost a dead hang with your body weight before failure, however, on your initial set, then your body weight alone isn't going to get you anywhere doing pull-ups. Weighted pull-ups should factor into your thinking, but you should approach them cautiously.
Based on my experience, starting off with a moderate weight increase (say a 10lb plate) and seeing how many you can do from almost a dead hang with that weight before you hit failure, is a good way to gauge your abilities and how much you can reasonably add on in 5 lb (or even 2.5 lb) steps to give you an appropriate resistance to where you can do 10 - 12 pull-ups before failure (this is the usual number of reps if you're looking for hypertrophy). Jumping up with a 25 or 45 lb plate right out of the blocks is a good way to trash your shoulders and permanently injure yourself or send you to rotator cuff surgery. If you weigh 175 lbs you can do 15 pull-ups and you start off with a 25 lb plate, you're increasing the weight by almost 15%. You might get away with this. 45 lbs is almost a 26% increase. Such a drastic increase from body weight is insane. Go easy, Sparky, build up slow and incrementally, increasing the weight gradually every other work out so your shoulders can get used to the new demands on them. Weighted pull-ups place a lot more stress on your shoulders to stabilize you during the movement because the weight can act like a pendulum and throw you off if you jerk or hyper-arch your back (which is crappy form, weight or no weight). You want to go smoothly up, get a good squeeze and go down smoothly. Weights can really make the "negative" a new experience. This works for wide and medium grips. Weighted chin-ups should be approached the same way.
Be patient, go for quality in each rep, and in about 60 days of diligent work you'll make serious strength gains and see markedly increased definition especially in your upper lats, rhomboids, lower traps and teres majors/minors.