Okay, that looks like an old answer and things aren't as bad in 14.04 as they once were. Yes, the monitor on the left is usually the primary monitor and it is set this way by default.

The unity desktop is actually called the ubuntu-desktop and the reason for this is because unity is actually a compiz plugin so you are actually using a compiz desktop. With that said, you can install compizconfig-settings-manager to achieve the settings you desire.

Either look for it in the ubuntu software center or open a terminal and execute the following commands to install it, whatever you prefer:

sudo apt-get update sudo apt-get install compizconfig-settings-manager

After installation, search for ccsm in your apps or type the command:

ccsm

to start it up.

Now, click on "general options" and navigate to the "Display Settings" tab.

For the setting "Overlaping Output Handling" you can set which output device (monitor) is preferred as the primary monitor. You can set this to "smart mode" to automatically choose the primary monitor, to prefer the "larger output" (bigger screen) or "smaller output" (smaller screen).

Also, you can go down to where it says "outputs" if they are both the same size and you can click on either one of the outputs which will open the option to move the device up or down . You can also choose edit and you should be able to manually edit the dimensions to change one smaller or larger.

ccsm settings and standard desktop settings are sort of like state and federal law in the United States: federal law (ccsm settings) always trumps state law (standard desktop settings).