I'm guessing that you're referring to chili powder when you say a "tablespoon of chili"?
Well, regardless of whether you're talking about powdered chili, or chili paste, or fresh chili peppers, the fact is that many people use specific types of chili peppers (or powder derived from different types of chili pepper) because of other flavors beyond the basic "heat." Some chili peppers are more "fruity" and/or "sweet," others are "dark" and complex, others are strongly "vegetal" or "grassy" etc. in flavor. Some of very aromatic; others less so. Similarly, chili powders can vary significantly in their sweet or roasted or smoky or whatever notes.
If you're adding a lot of some form of chili, you'll be getting some of these other flavors too. This is perhaps an advantage of adding a larger amount of a pepper type that's less spicy: you can actually taste some other flavor components of the pepper. If all you're adding chili powder for is heat -- and particularly if you don't really want to taste the pepper -- you probably want to go with the purest and most concentrated form of capsaicin, thereby avoiding most of the actual flavor of the pepper (or the peppers a powder is derived from).