I agree wholeheartedly with Yaah. Absolutely do a rotation at your top one or two places for residency. This will serve you two purposes.
1) You will "get in good" with your top choice program(s). Some programs will rank a person who did an externship highly just because they know them, even if their numbers are lower than some other applicants. In fact, there are a few programs that have so few spots (one or two), that only those applicants who have done an externship get ranked highly enough to actually go there.
2) You will learn tons about these programs, both good and bad. A program can hide major problems/deficiencies for one interview day, but it is difficult to do that for an entire month. It's really, really important in my opinion to find out if you would fit in at a particular program, or if you would be miserable there for the next four years. Spending a month with them is the best way to do that. I know you said you don't want to "blow it" by doing an externship at a place that sticks you in a corner and ignores you. You're right, it's a wasted experience in a sense, and pretty much sucks. This actually happened to me at one of my externships. (Pretty much ignored for an entire week, nasty uncalled for remarks about my home school made to me by a couple of attendings, among other things) Although it was miserable, it gave me some invaluable insight into the program, and so I knew better than to rank them highly because I could see the problems in the program. This saved me from making a grievous mistake, as I might have otherwise ranked the program first. I am actually very happy that I did the externship there.
The big question you might have, then, is which programs to consider as your "top choices" for residency at this point. Are the two you have listed (U of Rochester and Einstein) pretty high on your list of potential residency choices? If so, I would absolutely, positively do externships there. If there are other schools that you are contemplating, maybe you could list them and if someone here has done an externship/interview/etc there, they might be willing to PM you with more info.
By the way, Yaah. Yes you were very, very silly to not schedule a vacation for this month! I am currently on an "independent study" month. I am "independently studying" while waking up at noon every day, shopping, and hiking in the mountains. Tee hee! (Not to rub it in your face, or anything..
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Oh my, look at the time. I'm late for my Starbucks latte run! Ta, ta!