Contention rhymes with:
attention, detention, extension, hypertension, hypotension, pretension, retention, tension, apprehension, ascension, attention, comprehension, condescension, convention, detention, dimension, dissension, extension, gentian, henschen, hypertension, hypotension, invention, laurentian, mention, misapprehension, pension, pretension, prevention, reinvention, retention, suspension, tension
Contention sounds like:
condemnation, contamination, continuation