Great to include those options, because I think you'll notice why each site uses a particular choice. Let's break it down.
Twitter is notoriously up against a "What is twitter? How do I use it? Why should I use it?" problem. You'll notice that their design pretty much represents this. Your screenshot didn't include it, but I know below that header they have a bunch of grid-like stories laid out to try to communicate what Twitter is all about. That's great for them, and they're using their real estate most likely in the best way possible. I think you're right though, still requiring a click for either sign up or log in is harsh. They might have laid out the login fields for users who already have accounts and save them the grief.
Evernote. Chances are that if a user made it to this page, they have heard about evernote somehow and what Evernote needs to do is sign users up. They are in total "grow the base" mode and would love your email more than anything so they can continue to send you email campaigns and explain the power of Evernote to you. Defaulting to "Sign up" is a business reason, but they are going to annoy their users who have accounts, which is why I understand your desire to the merged form.
Facebook. Such a good example. Most of the online world has an account these days, so they cannot play any games with making it hard for their users to login. That all makes sense, users can use the top login fields without any extra clicks. If not, new users can create an account in the prominent and very visually different UI.
I would recommend something like this for your case. Use Facebook's template and the room on the left-hand side as a way to include your product highlights/features. There have actually been studies to indicate that following Facebook's design, overwhelmingly, because it is just so familiar to people actually makes for better usability.