Some table salts have iodine added. Not good when you cure with it.in my humble opinion, kosher salt, rock salt, table salt are pretty much the same....only difference is granularity of the salts... they all have diferent granularity, so one cup of table salt, kosher salt and rock salt have different weight measurement in one cup....whatever choice of salt you go with measure it by the weight not by the volume...
BTW....One of the most important thing before you even start process of salting is to squeeze out blood remains in vein that runs along pork butt.. do this by pressing with tumb along the blod vein... you will be surprised how much blood comes out....