When should we use what?

If you have a good thick  lawn already growing.

Use the aerator.

Follow up with a good program of lawn fertilizers. As your lawn continues to thrive, aerate once a year.

If your lawn is spotty, then use the overseeder

For a spotty lawn that is compacted or hasn’t been attended to in awhile, you might want to use both machines.

For a very poor lawn which has mostly weeds rather than grass on it, the best advice might be to just till up the whole thing and re-seed from scratch. 

I thought I was buying
Grass Seed 

I Thought I Was Buying Grass SEED
If you look at a bag of grass seed, it has information about the contents. Most people don't look at it, and that's too bad. That information tells you what types and varieties of grass seed are in the package, what percentage will sprout, the percent weed seeds, and the percent inert matter like fillers. Empty seed hulls, stems, and fillers can make up a significant amount of the material in a bag of bargain grass seed. How does it get there? Well, cheap grass seed is often not properly cleaned after harvesting it, leaving you with the dirt and other stuff that fill up the bag. High quality grass seed means high quality cleaning to remove empty seed hulls, stems, dirt and other material found in the field.