Neither. Plants utilize nutrients in the water which may be feeding free-floating bacteria clouding your water. But they don't do anything you cant do with a water change and you're going to have to feed the plants eventually which may bring the bacteria back, assuming the plants eliminate it in the first place. Plants won't do anything for other particulate matter that may be clouding the water.
Agreed, neither.
They do improve the water quality by removing nitrates and phosphates, and just generally giving the fish a nice natural environment, but as far as clarity goes they make no difference.