Apple Studio Display 21
Due to the requirement of on-board USB, the Apple Studio Display 21 can be connected only to USB based computers. An earlier Power Macintosh with a USB card installed does not work with this monitor.
Apple Studio Display 17
The Apple Studio Display 17 works with all Power Macintosh desktop computers, and all PowerPC based PowerBook computers with the exception of the PowerBook Duo 2300 and PowerBook 500 series with the PowerPC upgrade card.
For additional information on connecting the Studio Display 17 to older computers, refer to article 58271: "Apple Studio Display 17: Using With Older Power Macintosh Computers".
The viewable image size of the Apple Studio Display 21 is 19.8 inches, the Apple Studio Display 17 has a viewable image size of 16.0 inches.