TY: While most people stated that TY's were on average "cushier," they typically required more months of what I consider "hard" rotations (medicine wards,ICU,g-surg, ped's) than prelim med. Many of these TY's had fewer electives than their prelim med, however their elective months were unrestricted (meaning they got to do a radiology elective where they worked four hours a day during the elective). I believe all TY's required an emergency medicine rotation.
Prelim Medicine: Typically considered a harder year, with more months of medicine wards, but many rotations had more elective months than the TY year. The elective months were often restricted to medicine subspecialties (eg cardiology, GI, rheumatology) which certainly did not give people as much free time off as a unrestricted elective. Several of these places also required the interns to do one 1/2 day of clinic/week which made life more difficult on wards.
The general impression I got was that many of the TY's which used to be very "cush" have ended up increasing some of their required rotations, so you'll probably end up working just as hard or harder doing a TY. Basically the advice I would give is this: If you are going into a specialty which uses a lot of medicine and want a really strong foundation in internal medicine, do a prelim med year (more depth, less breadth). If you are going into a specialty which interacts with all services (eg radiology), you may want to do a TY year so you can see how they all think (more breadth, less depth).
Make sure to check with each program as to what an "elective" is, because things vary. Also make sure that they don't treat the prelim's in favor of their categoricals. I heard of one intern getting three "electives" in cardiology which was
Disclaimer: I interviewed for prelim's & TY's mostly in California, with a few places in the West and South. I have heard the prelim medicine years in the NorthEast (specifically New York City) can be brutal, making you do scut like drawing blood.
