They both have there benifits. In sand or mud, the traction control sucks as it limits the power when you need wheel speed. In rocks works great.
If I had to have one or the other I would go for the diff lock every time.
Used it heaps on the Merricumbene fire trail which is very steep tracks on loose shale. No wheel spin, no tyre damage, control all the way. I could feel the engine being limited by the traction control all the time. ASC off. Diff lock on, control over speed and spin, beautiful.