During the end of 2nd year, if you go to Sinai you will enter a lottery for the order and place of where you will do your third year rotations. Not everything during third year is at Mount Sinai but a majority is. I'll try and highlight the other main sites, pros and cons, just so you get an idea what they're like.
- Elmhurst - main affiliate site in Queens. You are required to do 3 weeks of general surgery here. You can also potentially do a month of medicine, peds, obgyn, psych, and neuro here. There's a shuttle that leaves to and from every 1-2 hours, so you never need to worry about commuting (it's a 25 min shuttle ride). There's also a subway if you want to take, which sometimes is convenient if you get off work right after the shuttle leaves and you don't want to wait for the next one. Elmhurst is Sinai's "Bellevue." Exact same feel, similar patient population, similar autonomy in handling your patients. Many of the attendings are Sinai attendings or did their training at Sinai, so same level academically as Mount Sinai hospital. It's a level 1 trauma center, which is why 3 weeks of surgery are done there so you can see and deal with true trauma patients. Pros at Elmhurst are: you deal with bread and butter cases (patients are less complicated and you usually deal with 1-2 major problems per patient, unlike at Mount Sinai where patients can have 5-10 major problems), tons of autonomy to manage your patients and do procedures. Cons are: less guidance from attendings, less devoted teaching time than at Mount Sinai, and potentially a higher workload if you get unlucky. I found Elmhurst to be an awesome place to learn for a student, so I opted to do a couple of my rotations there. But I still think Mount Sinai Hospital is the best place because of the mix of autonomy and guidance/teaching from attendings. Elmhurst suffers from the same problems as Bellevue - great place to manage your own patients yourself and do procedures yourself, but lacking in the academic and teaching areas.
- Bronx VA - nothing required there, but you can do a month of medicine, psych, and neuro. Similar shuttle situation as Elmhurst, so you never have to worry about commuting. The pros and cons of the Bronx VA is similar to any VA. Great place to learn bread and butter cases (especially for neuro and psych), easier workload, better hours. Cons are: being in the Bronx (so you won't really go out and explore, like you would at Elmhurst which has THE best places to eat), patients are largely male. It's a good experience to at least do for one month so you get a feel for what a VA hospital is like, but I don't think it's as good of a place to learn compared to Mount Sinai or Elmhurst.
- Morristown, NJ - can potentially do a month of medicine, obgyn, peds. You basically have to live at NJ for the month, although only like 2 people go there per rotation so you can avoid doing it at Morristown if you don't want to live away. They do not have daily shuttles (because it is >1 hour away) but provide shuttles to bring you to and from during the weekend. They provide free housing at Morristown for the month. Even though this is the furthest place to potentially do a rotation, they are desirable because it is by far the easiest place in terms of workload and hours and an easy way to get a good grade. So if you absolutely know for sure you don't want to go into a specialty like obgyn or peds, it's a great place to do it at.
There are also some other minor places that you can do rotations at, but I wouldn't worry about them. Generally it's a pretty good experience to do most of your rotations at Mount Sinai but be able to experience other hospitals like Elmhurst (community hospital), Bronx VA (VA system), and Morristown (private hospital). Since every place has a shuttle system (except for Morristown), it's not inconvenient to get there if that's a worry.
I personally think NYU and Sinai have the best systems set up for clinical rotations in NY. NYU's is the same basically: Bellevue (community hospital), Tisch/Langone (like Mount Sinai Hospital), and Manhattan VA. Theirs are conveniently within a 10 block radius though, which is nice.