Frankly, I think it may be because an early review of the "artist" model by an "artist" indicated he liked the extra firm tip. As a result I think a lot of us just ordered it based on that recommendation.
Since both of my iFaraday styluses have the "extra firm" tips, I can't compare them directly to the "standard" model but as I recall, the manufacturer recommended the extra firm for those who tend to press heavily on the screen when writing. The problem, as I recall, was that one could compress the tip sufficiently to make the edge of the barrel contact the screen.
There were some other problems early on with the first iFaraday styluses that may have contributed to the development of the "extra firm" tip. But I believe those issues were related to some low quality "substrate" materials that left smudges on the screen. To his credit, the manufacturer replaced all those with improved versions.
Bottom line: The manufacturer is extremely responsive to users' requirements. If you're not happy with his product, there have been a number of reports of his going several extra miles to satisfy his customers. I'd recommend his product without qualification.
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I can't compare them personally. Most of the reports I've read indicate the Bamboo stylus works very well. But frankly, after using about a half dozen different styluses I've come to the conclusion that one can make almost any of them work if you're prepared to invest the effort in practicing with them. Conversely, no stylus is going to improve your technique if the talent isn't there. (I know this because my sketching is just about as bad with one stylus as it is with another.

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The one advantage of the "artist" model for those are accustomed to a paint brush is that it is long and thin and "feels" more like a brush than the dimensions of the Bamboo stylus suggest.