I assume the latter routing is actually EYW-MIA-BOS. That's hidden-city ticketing or "skiplagging". If you do it, any subsequent flights on that ticket will get cancelled. Airlines forbid it, but they generally won't do anything unless you do it too often. The real risk is if they can't route you to MIA for some reason; they'll cheerfully route you through CLT to BOS and now you've got a big problem.