Android sensors and phone orientation

Hello, as you could notice this is my first post here.

I'm going to make some vision based navigation with an android device (using it's camera and if possible it's sensors). I will need to use a sensor to detect the orientation (or the change on it), I think that the digital compass should be enough to know where it's heading, however since the phone should be in a position so that the camera heads front I doubt that the information from the compass will be accurate. I don't know if a gyroscope or an accelerometer could help me with this problem.

To recap: The phone should be perpendicular to the floor (it can be either horizontal or vertical position), and I need to know the orientation of the phone.
Can I know this with the phone sensor?, does it need a gyroscope to accomplish that? what kind of information and in which axis of the phone can I get from and accelerometer in an android phone?

BTW: which one (Atrix 4g or galaxy S captivate) is a better choice in terms of sensors?

Sorry for my english (any correction would be helpful).

Thanks for your help.