One shape doesn’t work for all riders, which is why we offer more than one option.
The simplest way to choose between the two is to decide whether you prioritise ‘storage capacity’ (Half Frame Bag) or ‘water carrying ability’ (Wedge Frame Bag), as each size is more well suited to one of these than the other. See the below diagram to explain:
This is not prescriptive, you can of course always size up or down to get to your preferred frame bag size, but our truncated wedge-style Frame Bag would usually allow you to fit a larger bottle on the seat tube.