Poseidon all the way! I train a lot of rescue squads and they all choose to go Poseidon as well. In fact I was training 2 weeks ago in sub 30 degree water. My breath was freezing on exhalation and making pinging sounds inside the 2nd stage -- I was even able to get some frozen pellets back into my mouth on inhalation (I had just finished drinking A LOT and my breath was extremely wet)... Not one sign of freeze up or freeflow! I do use environmental kits when training in overhead super cold environments -- but this was a shallow training dive that I didn`t even use the environmental kit on! If I can help you with regulator selection or any other info -- please shoot me a message.... All the best- -- J