SCSS vs LESS

gistfile1.txt

LESS:

.kolor {
  color: red;
}
.tlo {
  background-color: yellow;
}

.klasa {
  border: 1px solid red;
  .tlo;
}

Generated CSS:

.kolor {
  color: red;
}
.tlo {
  background-color: yellow;
}
.klasa {
  border: 1px solid red;
  background-color: yellow;
}

SCSS:

@mixin kolor {
  color: red;
}
@mixin tlo {
  background-color: yellow;
}

.klasa {
  border: 1px solid red;
  padding: 50px;
  @include tlo;
}

Generated CSS:

.klasa {
  border: 1px solid red;
  padding: 50px;
  background-color: yellow; 
}

Sass is superior, it generates only what is used. Less is merging everything.

LESS:
.kolor() {
color: red;
}
.tlo() {
background-color: yellow;
}

.klasa {
border: 1px solid red;
.tlo;
}