When compared to your PRS-300 screen, the T2 and T1 models offer a 16-level gray scale E Ink Pearl display. This clarity and contrast of the image will be dramatically better than what you currently have. With plastic frames, the T1 and T2 are much lighter than the metal cased 300.
Bottom line - if you are satisfied with the 300 screen, size, and features, getting a backup unit may be an good option for you.
I have had numerous Sony readers over the years and have upgraded due to the need for better "readability."