I prefer pullups, did them for years and highly recommend them for widening the lats and overall back structure. I can't do them anymore due to rotor cuff issues. Also, even if I could, my bdwt now prevents me from getting enuff reps in. I believe pullups allows for so much more of a deep stretch at the bottom that it actually widens the scapula. Many have said the same, although its still a debatable topic. Most can't get quite the same contraction with pullups as with pulldowns unless they are stong enuff to actually pull their chest up to touch the bar. Even so, the phenomonal stretch and tension at the bottom to mid ROM is killa. I'd alternate pullups and pulldowns to get the best of both worlds. Of any bdpt, I often get asked in the gym how I built my lats, and pullups is always mentioned first in my answer.
p.s...One of the main problems you see with guys doing pullups is they only come down 3/4 of the way. Then they wonder why they still have no lats. They might be able to bang out 12-15 3/4 reps and they end up getting a bicep pump instead.lol They are missing out on a very important part - the bottom stretch - which also makes them harder to do.