Yes. What you wrote works:
<div [ngClass]="varA === varB ? 'css-class-1' : 'css-class-2'">
Plunker
The result of the expression on the the right-hand side has to evaluate to one of the following:
- a string of space-delimited CSS class names (this is what your expression returns)
- an Array of CSS class names
- an Object, with CSS class names as keys, and booleans as values
Maybe you had some other error in your code?