Hey, I can chime in with what my school says:
1.)If you're interested in a program, definately do an away there. Not only does it give you insight into a program (some people realize their 'dream' program is not all it's cracked up to be), but you will get a LOR from someone within the program. Also, although it's not universal, you usually do get an interview if you do a good job on your away rotation.
I was told by the program director at Montefiore that they'd rather take a student who may be a little bit weaker of an applicant who rotated thru their program than one who may be better but they have never seen (my classmates received similar advice from the PD's at Mt. Sinai, UCLA, and Cornell).
2.) We are told to do an elective, NOT a sub - internship. A sub - i may be more intensive, and could theoretically allow you to 'shine' more. However, there may also be other students of the home institution who can be doing a sub - i at the same time. These students will be more familiar with the hospital and have a distinct advantage over you. Also, in electives its usually just 1 student so you get more face time with the attending, and theoretically a chance for a better LOR.
Hope this helps.