Hello fellow Blender sculpters!
How do you prefer to handle details like hats, hair, clothing etc? Is it preferable to keep them on the same object if possible or is it better to split them up?
I would probably use retopo to start a separate object for the hat. It just seems easier to focus when you isolate elements sometimes.
Definitely multiple objects, since you can get to a really high polygon count by managing different objects on separate layers.
Also, as @Modron said, is easier to focus on one thing at the time when you can isolate the elements, and, if you plan to retopo your sculpts, is also easier to have separated elements.
The only situation in which I would use a single object would be a practice sculpt, or if I’m trying to work in a more “traditional” or classical style.
Thank you both for your answers 
Yes, I tried to retopo the face and it became hard to do a good work. Also, eyes and other landmarks can be made immutable.