I like both .... Twitter gets you direct access to people who are interested in whatever you're offering. From there, you can transfer those followers who are engaged to Facebook to learn more.
The more the merrier! Always aim to maximize marketing efforts of all kinds and by way of all social media opportunities.
It all depends on your followers. Both can be great tools for marketing and keeping your customers up to date if used properly. Why not use both, unless your whole business revolves around facebook and twitter.. If that is the case I cannot help you other than give you some advice a wise affiliate manager gave me when I was learning affiliate marketing "Get off of Facebook"
While Facebook is definitely much more common (billions of users compared to twitter's millions); however when it comes to "post visibility" and, hence, engagement — twitter is better. If you post something on twitter, you can be SURE that people are going to see it. From that point, it's just up to you to make sure they want to click on your links. Also it's free, while with Facebook, if you're not investing money, you're essentially wasting your time.
I think that it depends on aim of using them, but these sites are the most popular social media sited today and if you use smm for business it's better to use them both.
I love the idea of using them both. For me, each of them are effective. I love using Facebook because of its features such as the fan page, groups, etc which is indeed a big help towards the success of my website. For twitter, I enjoy tweeting and using hashtags to promote my website.