i own both the plain and combo edge Leeks, and i've purchased the Scallion and the Chive for my father and wife, respectively. they are all beautiful knives, and the SpeedSafe mechanism is outstanding.
admittedly, i purchased my first Leek (1660ST) solely based on the aesthetics and the wow factor regarding the SpeedSafe action. luckily for me, what i received was a quality tool, and is now my EDC, both at work and at home. for its size, the Leek has a very reassuring "heft" to it, yet rides so slim on my pocket, i often forget i'm wearing it. the serrations on the 1660ST were a bit disappointing, however - perhaps it's the nature of the cut, or the blade thickness, but they're a bit crude on the edge.
i alternate between the thumb-stud and the index-finger "trigger", and both work fine. on a somewhat unrelated note, i seem to recall someone asking in another thread about the ease of opening/cleaning a Leek. i can say in all confidence that this task is safe and super-easy. i have opened and cleaned mine a couple times and the "pivot-screw" still tightens and adjusts nicely. the only aspect of the Leek which i dislike is the lock-tab. i find it incredibly "cheap". it has a tendency to lock and unlock when i don't want it to, despite some subtle adjustments with a torx driver. it finally drove me to the point of idiocy: i've been carrying mine without the lock for months now, without incident (something nasty is bound to happen, more on that as it develops!).
my father works in receiving at a warehouse store, and uses his Scallion an average of 20-30 times a day. not even a week after receiving his knife, he told me 4 of his fellow workers had ordered a SpeedSafe! his has seemed to hold its edge quite well, and he prefers the mid-size to the Chive or Leek. definitely something you'd want to consider. another factor is materials. the Scallion thus far is only offered with Polyamide or Aluminum handles, with steel liners. there doesn't seem to be a stainless steel version (anyone know why not?).
my wife probably only uses her Chive once or twice a week, but seems happy with it. i personally find the Chive ridiculously tiny, and as you've certainly read, the thumb-stud serves no purpose other than providing a positive stop for the blade. i'm not a big fan of the curvaceous blade on the Chive and Scallion, but i can certainly see the practicality in it.
having never handled the Boa (though this will likely be one of my next acquisitions!), nor any of the other SpeedSafes, i can't comment on them.
hope this helps (sorry if it turned out a trio of mini-reviews).
abe m.