They have a quite detailed drawing that shows cross sections of both heads (either on their blog or the Genesis section, I can't rember)
You need to decide if you want extra material on the end (potentially lasting longer) or you want to be able to scoop more.
Edit - Andy found it…
@moog started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
They have a quite detailed drawing that shows cross sections of both heads (either on their blog or the Genesis section, I can't rember)
You need to decide if you want extra material on the end (potentially lasting longer) or you want to be able to scoop more.
Edit - Andy found it…