Either will get you your IR certification www.nachi.org/ir if that is your goal in taking a course. Because of InterNACHI’s prominence, Infrared Certified is what consumers inquire about. InterNACHI gets bombarded by consumers asking if a home inspector is Infrared Certified.
Conversely, the Level thing (Level I, II, III) is an unknown designation outside of the inspection and IR industries and carries no weight with consumers. It’s definitely more robust, but consumers never ask about levels. I can honestly say I’ve never had one consumer or agent mention the word Level. However, if acquiring different levels is your goal, and you plan on achieving further levels, Infraspection is the way to go and you should likely start with Infraspection to begin with.
They are each taught using very different styles. I prefer John’s style myself, but it’s a personal preference.
I hope that helps.