Like some of the other posters said, it's often about what kind of operation you're running. If you want to develop a deeper relationship, and have more involved conversations, with your (potential) clients - then Facebook is better. So, for example, you've got a business selling solar panels. Facebook is a goldmine for those kinds of "alternative lifestyle" groups - and if you're savvy enough, you can both engage with people by showing them what you've done with solar panels, as well as point them in the direction of your own products. And it can be "official", as in, "Hey, I've set up a solar water heating system. By the way, I sell that same system on my site, if you want to check it out..." OR you can do it unofficially, by setting up a Facebook page for "Solar Water Heaters Inc.", and then posting under your real name, sharing information, and then mentioning that Solar Water Heaters Inc. has a good deal on such-and-such products.
Whereas Twitter... It's social in a different way. I think things can go viral on Twitter much faster than on Facebook, but the 140 characters really limit the depth of your interactions. It works for me in two ways; linking to articles (on news websites, blogs, whatever) and just silly banter with followers. That's nice, too - for example, you won't get big walls of text directed at you if someone disagrees with you, like you can get on Facebook.
Myself? I prefer Facebook. It's better for my business, since there are more seniors on Facebook than on Twitter, and my products are more expensive. Older people are more likely to have the extra cash to spend, hence my focus on Facebook.