Good to see so many responses point out how both of these extremes as defaults would get crushed over the long-term; poker is greatly about balance and adjusting.
However, all else equal, I'd say that typically under-representing the strength of your hand is more likely you'll get paid when they call, but over-representing the strength of your hand is likely to win smaller pots more often (as others fold). Both played as a strategy are doomed to failure, but the art of adjusting when correct is a critical factor for success