Right from the start, the two titles and their function is different. While "Foe" is a short, posh word, that sticks the reader's mind and attracts him/her because it is unnatural, "Robinson Crusoe" mainly focuses on the protagonist himself as if the novel his biography. In addition to this Foe is an umbrella term, so it can have a lot of different meanings; such mystery attracts more the reader.
Foe is made up of only 4 chapters, and it is generally shorter than Robinson Crusoe. In particular Robinson Crusoe had got 46 chapters that contain the protagonist's life from his childhoodto hid going back to London, after 27 years of loneliness.
But the meaning of a book is built by the reader, not by the writer. For this reason the ideal reader of the two books is very different, not only because of the different ages. Robinson Crusoe was thought to be a children's book, or at least a post-colonial novel to enjoy people from differetn XVIII century social classes with descriptions of exotic places and animals. So it has remained very popular among people till today, while Foe won't be so popular because it is much more complex and it requests a notable cultural background to be understood.
Coetzee is a post-modern novelist, and his novel contains lots of themes and issues that in Robinson Crusoe couldn't be raised up. Travelling was an adventure and not a common thing, while today (Foe was written in 1986) the world is a global village in which everybody can talk with everybody on every moment, and in which sharing toughts, actions and feelings is absolutely common. Incapacity of communicating, racism, woman's condition are themes that in XVIII century couldn't be found in any book.
Another important dissimilarity between the two novels concerns the narrative techinques. In Robinson Crusoe the narrator is Robinson himself that speaks about his life and adventures just like in a sailor's logbook. In Foe the speaking voice is Susan Barton (a woman: another relevant difference): iti is right to say that she is the reader's eyes and hears, bacuse through her thoughts and considerations the reader imagines situations, but he/she is not free because of Susan's personal point of view. The role of Friday changes in the two books: while in Robinson Crusoe he takes part in the adventures of his master but he isn't so relevant in the balance of the story, in Foe he doesn't seem to be able to communicate his feelings but this lack of "voice" is one of the central points of the novel. In Robinson Crusoe there are a lot of descriptions of places, situations in a very detailed way, while in Foe there are no descriptions but only descriptions of Susan's feelings and thougths.
Robinson Crusoe was a great success in his age because it could be read by people from every social class, Foe doesn't have this feature, because the concept of society has changed and the expression "social class" has now a different meaning.
Moreover, Coetzee is from Sudafrica, in which apartheid was a strong reality until 1994: the problem of racism in our age has a different conception from the one is had in XVIII century, in which black people could only be slaves. Foe goes beyond this idea and it shows Friday in a mych more important position.
Foe is much more complex than Robinson Crusoe because the circulation of culture is easier nowadays, so common people read more and "further".