In regards to the fixed pin 510, while they're calling it "flexible", I believe what they mean is that the center pin insulator has a bit of give to it. But be warned, I've had a couple of mods of which that flexibility developed memory and wouldn't make contact with shorter protruding tank contacts anymore. Just bear in mind if you're really trying to torque a different tank down that doesn't want to sit flush.
Dang, this thread has really gained some traction.
I see this as a fairly surprising flaw as the floating pin seems standard these days but I'll be using it almost exclusively with the subtank it came with so it is a flaw that won't impact me.