Any door or window that swings is going to be able to get a better seal than one that slides. There's a reason all the hatches on the International Space Station swing open.
Also, personally, I can't stand sliding doors; the motion of opening and closing them is unintuitive. And the bottom trough it slides in gets gunked up because you walk over it, resulting in another thing that has to be cleaned and maintained lest the movement of the door becomes rough/sticky. Compare to a swinging door, that is typically so maintenance free you don't even think about it for the lifetime of the door.
I'm stuck with sliding windows and a sliding patio door because that's what my house has and they were all pretty new when I bought the place, but if you have an opportunity to replace a door and make an informed choice, I strongly recommend a swinging door over a sliding one.