It depends on personal preference, really. You can always do split testing to see what works for you. Facebook has already won the competition with yahoo in terms of ads. I like Facebook as it tend to become more personal than twitter. Rain
I think both can help if you know how to. I just got a lot of traffic few days ago because of single tweet and I have seen an example of dramatic SERP rising with tweet from account with several thousands followers (first page result on Google for new site and highly competitive keyword) but for only one day. I have also seen very powerful links from FB profiles (yep, do-follow) for better PR and SERP. So both could be useful. I am not expert for these kind of help but believe it takes some time and knowledge.
Facebook is good for long term connection with customers and twitter can generate unique visits to you website
Twitter is the best. I your are looking for web traffic from Social Networking Website, Twitter is better than Facebook. But If you are looking for the chat and other friendship activities the facebook is best.
Facebook is more worldwide compared to twitter. Twitter is still not popular in some countries, while facebook is popular in most of the countries.
Use them both together. Your accounts can be linked as well. It's all about creating an online presence.
Facebook is definitely the better choice although a downside would be high price of the facebook ads if you want promote that way.
Thank me later for reporting your spam to Smile Now! Just integrate the social app's and plugins into your site and see which one works best, Tweet automagically to Twitter through posts (if using wordpress) and use facebook comments, likes and shares etc. in your site and if your site and posts are good enough - then you'll have the work done for you - as your visitors will share your shit for you. If your site, business, posts - whatever you are selling is shit - then nothing will help you.