[snellemin]

Simplest way to explain is that the 1520 just makes more torque vs the 1515........

With the same continuous load the 1520 will waste less power in heat vs the 1515. So for 30A load, the 1520 will make more use of it vs the 1515.
See what works for you man. Sometimes the lighter weight of the 1515 is preferred for better handling.