Well the Sears 9mm borescope arrived just yesterday. My initial impression was good, but it doesn't come with a protective case like the Harbor Freight unit does. I installed the four AA batteries & played with it around the house for a few minutes. It doesn't have as strong LED lighting as the Harbor Freight (HF) unit & the HF unit seems to have a better picture quality. I suppose that stands to reason that the unit with the bigger camera lens would have the better picture quality. The Sears unit will fit down a sparkplug hole on even the small diameter plugs where the HF unit will not as it is a 16mm or .630" end on it. The Sears unit also does not have any provision to install a plug jack & hook it up to a larger monitor or a recording device like the HF unit does.
So there you have my very informal notes on the two units. If you want to go through a sparkplug hole to inspect a cylinder, then the Sears unit is the way to go. If you have an entrance that a 16mm end will pass through, then the HF unit is better.
Good luck
J.