You can see on the left the core aerator actually pulls soil cores out of the ground. As a result it is much better at relieving compaction than a slit-seeder. As a result, it is commonly used by landscapers for seedbed preparation during fall over-seeding. They then come behind it with a broadcast spreader to apply their seed and fertilizer.

The slit-seeder is on the right. Instead of taking a core, it has disks on the back that crimp a “slit” into the ground. The hopper on the top is full of seed which is fed through tubes down into the slits where a roller behind it rolls it in. Its main benefit is that it is a one-pass method which saves time and labor. It is also very useful on slopes because it places the seed in the ground where otherwise you may have to worry about broadcasted seed washing away.

I would say slit-seeding is more of a specialty service that some landscapers offer where aeration is much more commonly done. They are both very effective though.