As far as taking the general or mechanical section in the afternoon... I took the mechanical. I also heard that the general was easier. I thought that I found pass rates and they were better for general than for mechanical, but I don't remember for sure.
I'd say study for both, and then decide after the morning session. If you weren't confident with a lot of the mechancial questions in the morning session, go for the general. I thought the mechanical questions, in both the broad (morning) exam and afternoon mechanical section had more of a focus on thermodynamics, as opposed to being an even variety of different subject areas. This may work out for you, or not. Or maybe when you get to the test, many of the mechanical questions on the morning section will be about fatigue life or something.
Use the mechanical questions of the morning exam to decide if the focus is on something you are confident with (or if it is varied) and base which afternoon exam to take from the morning session.
-- MechEng2005