Ah yes, the time-honored track selection question. Bottom line: It don't matter. Seriously.
Every rotation is new and while there may be overlaps (OB/Pedi, IM/Surg, Pedi/IM, Psych/Neuro, FP/Everything), each rotation is different and requires different knowledge base/skill set. I agree, the further along your 3rd year, the higher the expectations. At the same time, you're more comfortable with the way things work the further along and have less self-esteem issues.
In terms of looking good and getting good grades, the most important thing in 3rd year is to put forth your best effort in your work, your reading, and your relationships with people (patients, residents, students, attending) because people will expect you to be as smart/good as a 3rd year student at that point in time. Realize that your 3rd year grades are based on your book knowledge, clinical skills, and your personal/work skills.
If you have a rotation in a field that you're interested in but it falls early in your 3rd year schedule, you can always use the 1st 3-5 rotations of the 4th year to go back and make an impression, get some letters, etc. Otherwise you can use your last rotations of your 3rd year to pick up letters.