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@Kaden Alexander they are less popular cos they are rarer and more expensive (usually at least twice the price) so if people want a big impressive brown spider, then stirmi is the more popular choice. there are 3 therephosa species in order of rarity and generally price, they are:
T. apothysis
T. Blondi
T. stirmi
The main difference between Blondi and stirmi when young is that stirmi have pink feet, Blondi don't. When adults the only real way to tell a Blondi and a stirmi apart is to look at the knee joint. Blondi have additional long hairs on the knee, stirmi do not. Unfortunately due to the higher price a Blondi commands, a number if stirmi are actually sold as Blondi or under the common name of "Goliath Bird eater" so you do need to be careful with these.
Hope that helps. BTW, I have 4 of each Therephosa species at various life stages so can share other pics and info with you if interested. there is a collage I did of each sling in the gallery too.