What are the similarities and differences between Mars and Ares?

They quite literally ARE the same thing under a different name. The Romans simply adopted the Greek Pantheon and changed almost nothing but the names. I guess the only real difference you could say is that the Greeks did not like Ares as he symbolized more the negatives of warfare (while Athena symbolized the good) while the Romans were a much more war faring people so Mars was worshiped to a greater extent than Ares ever was.

There aren't any differences or similarities, they are the same mythological being with different names. Are you sure you understand that?