AFAIK you can only do this using UI scripting, here are some ways to switch to the miniplayer : 

Simulating a click on the button in the interface : 

    tell application "System Events" to tell process "iTunes"
    	click button 4 of window 1
    end tell

Using the keyboard shortcut : 

    tell application "System Events"
    	tell application "iTunes" to activate
    	keystroke "m" using {option down, command down}
    end tell


---
If you want to display the miniplay AND the fullsize player (your request isn't clear) use : 

    tell application "System Events"
    	tell application "iTunes" to activate
    	keystroke "3" using {option down, command down}
    end tell