Directly switching (without pressing OPTION) between AIFF and mp3 iTunes libraries
Try this.

Unfortunately, there is no direct method to use AppleScript to load a library, so this method instead adjusts the the iTunes preference file via AppleScript. I just verified that the method works in 10.8.4.

You'll want to follow the procedure once for each library, then export as an application from AppleScript Editor. Once you create the applications, you can simply doubleclick on the appropriate application to load your chosen iTunes library.

I made one slight addition to the AppleScript in order to make it the frontmost application:

tell application "System Events" to set frontmost of process "iTunes" to true