Hey there,
I haven't heard of any sites as being the bomb for ER, but I can give you some pros and cons regarding hospital vs preceptor:
Good things about hospital = don't have to learn new systems every month, can get to know people and procedures well, don't have to drive all over town.
Neg. things about hospital = have to put up with the same people, possible to get a bad reputation, don't have as wide a variety of clinical settings.
Good things about preceptors = new start every rotation, even if you hate the guy it'll be over soon, get to see lots of things.
Bad things about preceptors = stress trying to figure out where the heck the doctor's office is, not enough time to build a strong relationship, and if the doctor becomes unavailable for some reason (which has happened), you have the stress of having to establish a new rotation.
Hope this helps. But really.... it's all about the location.