Purples are nicer IMO, though they are reputed to be the most aggressive of the 3 species. Mine splits a fin on the Hector's Goby every so often, but no lasting damage (and he doesn't give chase). I don't think I would pay the money they want for the Helfrich's, even if I could afford it... though I understand if you get the Purple Firefish, you need to get a known pair to keep more than one?

From what I can gather, the other 2 species can be kept in groups, perhaps that's why these command a higher price (better disposition toward conspecifics)? I know the Purples are collected at a relative depth, it's likely that the Helfrich's is collected even deeper and require decompression on the way up (driving prices up due to longer dive times and higher mortality rates).