As adjectives the difference between marginal and borderline
is that
marginal is of, relating to, or located at or near a margin or edge;
also figurative usages of location and margin (edge) while
borderline is nearly; not clearly on one side or the other of a border or boundary, ambiguous.
As nouns the difference between marginal and borderline
is that
marginal is something that is marginal while
borderline is a boundary or accepted division; a border.