I would go with the Herpstat II for now.
Helix is doing a big redesign and so I would wait for the new ones to come out if time were on your side.
I have both Helix's and Herpstat's and they are both terrific units and Helix and Spyder Robotics both have great customer service (Dion at SR and Jeff at Helix).
I would get the Herpstat II over the Pro because it can handle higher wattage. I would get the II over the ND because you might want that feature in the future.
I think it's a good idea to put hatchlings in a 6qt tub as I feel it increases the chances for promoting excellent feeding responses