Apart from the obvious 2012 booklet and 2013 pane format and date (the latter with perforations all around) I thought I'd scan the two stamps next to each other to see if there were any other noticeable differences:

As expected, they are pretty much the same, although I do detect in the 2013 stamp a slightly bolder print in the word "Christmas" and it would seem there is an extra space between "USA" and "FOREVER".
There is also a slight difference in the vertical centering of the stamp, most noticeable when comparing the lettering to the frame of the stamp, but I believe that is probably within acceptable printing tolerances.