The 5SY5103-6CC12: The B-curve trips between 3 and 5 times rated current (9–15 A), which is the standard choice for resistive and general-purpose DC loads where you don't expect high inrush.
Single-pole width saves rail space compared to a two-pole DC breaker when you only need to switch the positive leg. The 5SY5 series uses the same tunnel terminals as the rest of the 5SY line — wire it with the same ferruled conductors you'd use for the AC branch breakers alongside it.