I have always preferred wide-grip pull ups. I feel that it works better. I sometimes use pull downs for burnouts with lighter weights after my pull ups. I feel that pull ups are great for functional strength as well as for back width.
Hi! You should concentrate on making wide-grip chins the staple movement and then do pull-downs for variety every once in a while. The wide-grip chins are a more involved movement (and harder) working more back muscles than the easier pull downs. If you can work yourself up to doing 3-4 sets of wide-grip chins for 12-15 reps on your last set, then believe me, you will be well on your way to having an awesome back!
I hope this will help you out! For more fitness tips, advice and info, feel free to visit my personal training website @ http://members.shaw.ca/muscleman/