The question is actually more javascript animation vs css animation (because angular2’s animations are based on javascript-animation).
The answer is that when you can – use CSS animation.
Modern browsers uses different thread for CSS animation, so the javascript-thread is not affected by the CSS animations.
You can use the HTML5 Animation Speed Test to check the preformance of different frameworks (javscript-based) VS CSS animation in your browser.
In general:
Browsers are able to optimize rendering flows. In summary, we should always try to create our animations using CSS transitions/animations where possible. If your animations are really complex, you can may have to rely on JavaScript-based animations instead.
If you want to know specifically regarding the Angular2 animations – just inspect the element in your browser and check if the animation there is a CSS(transition/animationFrame based or javascript (you will be able to see values in the style attribute change during the animation).