THere is no difference in the offerings of the 12' and 13'.
What LR is trying to say in their marketing material, is that they have rewritten some lines of code, or changed suppliers of certain control modules to address concerns of the 12' owners (this is normal for any manufacturer). This in no way shape or form effects the functionality, with the exception of maybe a little faster less glitchy system.
That being said, if you had a 12' and a 13's side by side, and similarly equipped you would not notice a difference in the functionality more than likely.
Save the money buy the 12', as I always say "you'll never know what your missing because you won't have it"