Generally, we compare things that are similar.
So, you would not compare a taco to the "American one".
If there are two types of tacos, American and Mexican (which is a bit weird since tacos are originally Mexican), you would say:
- The American taco was spicier than the Mexican one.
The is used because you have just eaten those two types of tacos. They are specific, and you are not making a general statement about tacos.
As for sushi, it really is only Japanese. So, to compare like things (fish), we would say:
- The sushi was less expensive than the salmon. [on a menu]
The rule about article the here is that you are talking about a specific food item you have just eaten. You are not making general statements.
Sushi is less expensive than salmon. or Tacos are great.