Dave:
The "6B" selector valves have an overlap so that if you shift while pressurized fluid is flowing into the SV and out a work port (you are extending a cylinder) there is at every moment during the transition a place for the pressurised fluid to go (i.e., to extend the cylinder in the other circuit). This avoids a sudden stoppage of the pressurized flow and the resultant pressure spike. It is just like the water hammer in home plumbing when the faucet is suddenly closed at the end of long piping; the kinetic energy of the moving fluid creates a pressure spike much higher than the normal system pressure. This pressure spike may ultimately trigger the system relief valve, but only after it reverberates off the closed selector valve and travels back to the relief; in the meantime the pressure spike can damage hoses, etc.
The disadvantage to selector valves with overlap is that if you are using the valve to actually hold the cylinder extended after the work ports are shifted to the other cylinder, there will be pressure from the extended cylinder that may overcome the pressure (if any) in the other work port while they are connected, which would allow the extended cylinder to fall a bit and force fluid out the other work port. As a practical matter, selector valves usually have too much leakage even when closed to hold a load in position for very long.
So the "6A" selector valve with no overlap should work fine if you remember to shift it only when no fluid is flowing from the pump port through a work port. If the SV is in its typical location downstream from a directional control valve, then the DCV should be in neutral, so that no fluid is flowing to or from the SV, when the SV is shifted.