both seem to be ok and of newer vintage ..meaning with a good handlepost versus the old one
not too much difference between the old like 3to4 years old mariner and the 2to 3 year old boardwalk ...wild guess

current boardwalks have one gear and lesser handlepost ..current mariners are real nice alloy frame and much better parts but also 500 plus new ..

for a used bike these seem to be a little pricey but maybe worth it... offer less and see what u can get