battery size with the RT being 31.5 W-h and the Pro at 42 W-h. There are also a couple of other tech differences in the hardware with the RT being 32 or 64 GB and the Pro as 64 or 128 GB; RT has only USB 2 while the Pro has USB 3 - - why put out a new device as only USB 2?? Pro also comes with a Palm block. Weight RT 676 g - Pro 903 g. Those are the main spec differences of the hardware.