City directories generally list heads of households and occasionally their spouses. The 1950 census listed everyone by name. Remember that when using the 1950 directories, they were not created to list everyone.
Keep in mind also that the city directories currently on Ancestry.com generally are for larger urban area. That leaves those of us with rural ancestors out until the actual census is released. And the 1950 census is not scheduled for release until 2022. That's just a little while--but I still won't be listed.
The image shown in this post comes from the 1951 directory for Rock Island. The entry at 1207 is my wife's great-grandfather, Henry Mortier. I'm assuming the little "bell" means he has a phone. The circle thing I'm not certain. My quick scan of the front of the book found a list of abbreviations, but somehow I missed the symbols. If anyone has them, let me know--I'd like to post them.
The "census substitute" is a great source--but just remember what it is and what it's limitations are.