I like Twitter a bit more then Facebook just because it seems that you can sort information faster and get messages out quicker. Now that Google+ has added Hashtags I think it also should be included in the mix of best social networks. Google+ seems like it might be turning into the best of both Twitter and Facebook combined. I also like the freedom and simplicity of Twitter where Facebook seems to restricted, cumbersome and regulated at times.
I think both are effective to generate traffic and leads. If efforts are made in the right direction, Facebook & Twitter can both equally be successful to generate traffic to your site. Apart from this, both sites provide paid advertising as an alternative to Google paid advertising and many of the websites benefited more than Google paid advertising.