When choosing the right double-12 tanks, you can choose between a round bottom and a concave bottom. Bottles with round bottoms allow for a more central weight distribution, while concave bottoms allow an easy bottle placement and prevent a top-heaviness of the diver.
Furthermore, there is a choice between Halcyon and Scubatec valves. Halcyon valves are of a higher quality and smoother running than Scubatec valves, TÜV and similar approvals have of course both. For further information please refer to the individual product pages: