Alien, the difference between the dome and the ridge...

The most common theory I've heard about this is that the dome is shed after a few weeks.

Either that, or the ridge heads are warriors while the dome heads are "workers", commonly referred to as "drones". The movie AvP Requiem tried to introduce the notion that the warriors come from a different kind of facehuggers, but no one cared because it was a stupid movie.

I like the smooth head better. It's looks cooler.

Smooth dome!