From what I've read, some people have reacted poorly to both substances. Sodium lauryl sulfate is definitely the harsher detergent of the two, but sodium laureth sulfate can also cause skin and eye irritation, especially with long-term use. My shampoos and facial soaps in recent years have contained sodium laureth sulfate instead of sodium lauryl sulfate, and unfortuntely skin irritaiton persisted for me during use.
You might want to check and see if either ingredient is in your shampoo. If you've never experienced any major hair or scalp problems while using your shampoo, then you might be fine with a SLS/SLES product. Me, on the other hand... well, my hair problems were a mystery for years before I picked up on the SLS/SLES sensitivity issue.