Neither one is ranked above the other, and that’s the honest answer, not a dodge.
For an open surgical wound, you can use a normal saline solution (salt water) or mild soapy water. Either one. There’s no rule for which is better, and no situation named that would decide between them.
If you’re irrigating a wound with a syringe, the choice is your provider’s: fill a syringe with salt water or soapy water, whichever your provider recommends.
For other wounds, one or the other is used. For a stage I pressure sore, mild soap and water. For a stage II pressure sore, salt water, also called saline, rinsed to remove loose, dead tissue. Your provider may recommend a specific cleanser instead. For a wound with a wet-to-dry dressing, warm water and soap, then rinsed with water. For minor wounds, gentle soap and water.
Put together, there’s a pattern. Saline shows up where there is dead tissue to rinse away and a wound bed to irrigate. Soap and water shows up for ordinary cleaning. But that’s a pattern, not a stated rule, so don’t treat it as one.
What is firm is the other direction: what not to use. Do not use skin cleansers, alcohol, peroxide, iodine, or soap with antibacterial chemicals, because these can damage the wound tissue and slow healing. For most wounds, what matters is staying off that list, not choosing between the two options that are on it.
If you were told to use one of them specifically, that instruction came from someone who could see the wound, and it outranks the general picture here.