A Suggestion.
I think the only solution to this problem is simply going to near by Apple Store or Center to check the two different models out yourself.
The problem being, as I have noticed reading these guys posts, the ones who never had a chance to use the 14 inch, but first bought the 12, are saying that they would've went for the 14, because of larger screen size, and the ones who bought the 14, say they want the 12. This is obvious. Apart from those who really chose the ones they wanted, everyone kind of wants to try the other one just for the sake of it.
and QCassidy here saying that he prefers the 12, is because he owns the 14 as well, so if he wanted to use the bigger screen, he COULD. However, most of us did not have the option to do that, if we chose the 14, we were stuck with the 14, chose the 12, then the 12 we were stuck to.
But, as I get carried away here immensely, what am I trying to tell you is that, it is impertinent that you see the Books yourself. Try to see which one you prefer looking at, and if possible, try to use them for the maximum time possible.
If you get a small house, you're gonna want a big one, for spaciousness and airiness. If you buy a big house, you're gonna want cosiness and seclusion. Same story here. You might get frustrated with the 12 inch screen, and want the 14, or you might start to want the smaller cuter 12 inch version if you had to walk around with a 14. (The 14 inch is not big at all. BTW)
Check them out in real life. Both at the same time. Best way to compare.