Both, when viewed in photos or in person, exhibit no facial features.
Both have the word "Slender," in their names.
Both seem to be incapable of speech.
Both cause electrical interference.
Both appear to be capable of teleportation.
Both are shown to be very aggressive towards others.
Both can generate tentacles/limbs from their backs.
MARK SLENDER: A photograph of him shows he has teeth (assumed to be the only facial feature left due to the extensive damage implemented by the burns).
SLENDERMAN: His face does not possess any facial features at all; facial appearances always reflect what the individual viewing him sees as terrifying.
MARK SLENDER: He is considered an apparition of some kind.
SLENDERMAN: He is considered to be a more of a physical entity than a non-physical one.
MARK SLENDER: Wears a striped, twisted suit without a tie.
SLENDERMAN: Wears a solid black suit with a tie.
MARK SLENDER: Operates on a different plane of existence.
SLENDERMAN: Operates on our plane of existence, but in a different dimension.
MARK SLENDER: Glides above the ground and moves very swiftly.
SLENDERMAN: Moves very little, to never at all, especially when being observed. Many of his movements (if he displays any) are very slow and fluid.
MARK SLENDER: Limited teleportation abilities.
SLENDERMAN: Extensive, if not near omnipresent teleportation abilities.
MARK SLENDER: Explicitly has a backstory, explanation, and reason for being a malevolent and destructive entity.
SLENDERMAN: Currently has no definite explanation for his existence or behavior, and every medium depicting him either doesn't explain it fully or has a self-contained explanation that only works for that medium.