If you have premium status, you get better seats with more legroom. But those seats are the most coveted so you’ll almost certainly be sitting next to an occupied middle seat. But if you don’t have status you get worse seats (usuall near the back of the plane), but you have a better chance of not having to sit next to someone.
Bad coach seats with an empty middle seat is BETTER THAN a good seat with a full middle seat. So depending on when you are flying, you have to make a decision about how full the plane will be as you choose where you’d like to sit.