Def go for the digital one. I used to have a mercury one and it takes ages to get the thing to move up from when you shake it down, expecially on those cold winter mornings. I changed to digital (2 dec places) about 6 months ago and they are fantastic. I make sure that I leave it in my mouth for about 2 minutes before I turn it on though cos I found that is warms up the metal bit to what your correct temp is and is even more accurate then.
They can be quickly turned on later on when you are actually awake and can put it on your chart with wide eyes instead of sleepy stabs of the pen in the half light ( IFYKWIM )