There is little point in upgrading. 12th ed. is a perfectly good reference text to prepare you. In fact, since it has the practice problems right in the text, I'd say it's better for at least the reason you don't have to buy the separate text for them. If you use the MERM in conjunction with your undergrad texts you will have every underlying theory and methodology you will need to prepare.
Note: I began my study with the 12th ed. using a borrowed text. I quickly found that I wanted to make notes right in it, so I gave the borrowed text back to my friend and bought my own. Since I was purchasing anew, I decided to get the latest, but believe me, there is very little difference between the texts and I would have been just as well served to have bought the 12 ed. And there were more people in my exam that had the 12 ed. sitting on the table than the 13th.