This is actually a very interesting question, but one I fear has no right or wrong answer. Like THP said, you have niche within niche, heat levels are subjective, what one finds hot another will barely sniff at. Tastes vary, and food trends vary not only nationally (and internationally) but also by region. There is no guarantee that a sauce which sells well in Florida will sell well in New York and vice versa. Then you have to add in the marketing to the mix, your sauce may be the greatest in the world, but if the label has no curb appeal, people will not even pick it off the shelf to try.
We currently have 4 sauces, a mild, a medium(ish) a hot and an xhot, at the markets they sell pretty much evenly across the board over a period of time (yes I'm the geek that keeps spreadsheets to track). The gift buyers will typically purchase the hottest. At different stores across the country one will always out sell the other, this varies per region and per store, perhaps this may be more to do with the sales person's particular tastes than anything else. The majority of restaurants that stock us, stock the two hottest. Our website sales are starting to even out, although slightly tipped in favor by the two hotter ones.
Of course, we haven't even been in business a year yet (October) and two of our 4 sauces have not even been on the shelves 6 months, so take what I say with a pinch of salt
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Making the sauce is easy, selling the damn stuff....an intriguing game...