I think an away sub-I is a bad idea if you are fairly competitive, but an away elective on a subspecialty service is a great idea. If you do a subspecialty service you will likely work a manageable schedule with weekends off, no call, and work closely with an attending or two. Your exposure is more limited, so it is easier to learn and shine while there. Your time off on nights and weekends will give you a chance to see the city.
Keep in mind the away elective is as much for you to see them than for them to see you. On interview day, you only see a snapshot of a program on their best behavior which may or may not accurately reflect how things really are. I did two away electives, and it was invaluable in helping me make my decision. The one I thought I would love, I actually very much disliked after spending a month there. Ironically, if I would have based my decision on the interview day alone, I would have likely mistakenly ranked it number one.
I would strongly recommend spending more time at a program if you have the time and resources. Three years is a long time to be at a program you may not end up enjoying so I would try to get as much information about a program as possible before making a decision on a rank list.