Neither is essential, but I recommend both.
A 2nd monitor drastically increased my productivity during MS1 and MS2. Think about typing up notes, going through QBank, etc. and not having to flip between your internet browser and application. It's much more streamlined.
As anyone will tell you, a tablet is nice to have for clerkship because of portability and ability to keep textbooks/take notes. It's definitely not essential, but I get plenty of use out of mine, especially if thinks are slow that day and I've got extra time study. I can fit the tablet in my white coat and use it whenever I need it.
If you only want to buy 1 or the other, you can actually buy a tablet and use it as a 2nd monitor (albeit a very small one).