Seal-in versus a latch instruction: is there a real difference?

Yes, and it shows up on power loss and on E-stop, which is when it matters. A seal-in is a branch around the start contact that keeps the rung true through the output's own contact. It is ordinary logic, so it obeys everything else on the rung. Put a stop contact in series and it drops out. Lose control power and it drops out, and it stays dropped when power returns because the coil is false. A latch (OTL/SET) writes the bit and walks away. The bit is usually retentive, which means it survives a power cycle. On power restoration the machine can restart on its own with nobody touching anything. That difference is the whole argument. For anything that moves, use a seal-in, because "does not restart by itself" is a safety property you get for free. Latches are still right for state that should survive a restart: a fault that has been recorded and not yet acknowledged, a mode selection, a recipe number. Things that describe the machine rather than energise it.