What is better for the client side, an animated gif or the same animation done as a sequence of images with CSS ?
What's better, apples or oranges?
An animated .gif gives the author extensive control (frame rate, length, color profile etc.) but will often have a rather bloated filesize (for an image).
CSS animations tend to be more succinct in terms of data transfer, but require more processing by the browser (and are subject to inconsistencies between browsers/rending engines) and may not work on old browsers.
Do you think that CPU/GPU is more precious to your users than bandwidth?
How important is pixel perfect rendering?
Balancing these factors, or even better testing the performance of each, is the only way to know which is better for your specific needs.
If the prospective clients of your website/application will have a low bandwidth connections, you should consider gif over CSS animation. You can have control over the gif from your side and can even alter the output from that single file if you require without multiple file alterations (on the CSS file and the image files).