GIF and MP4 both show moving pictures, but they work in completely different ways — and picking the right one can be the difference between a 6 MB file and a 600 KB one. Here's how to choose.
GIF is a 35-year-old image format. It stores at most 256 colours per frame and compresses each frame more or less on its own, so anything with gradients, video footage or lots of motion balloons in size. MP4 uses modern video compression (H.264) that looks at how one frame differs from the next and only stores the changes, plus it keeps full colour. For the same clip, an MP4 is routinely 5–10× smaller than the GIF and plays back more smoothly.
If it's going on a modern website or social platform, use MP4 — it'll look better and load faster. If it needs to autoplay in an unpredictable place or it's a small simple loop, keep the GIF. You don't have to guess blindly: convert either direction for free and compare the sizes yourself.
pixelpix does both in your browser, with nothing uploaded: turn a GIF into an MP4 (or WebM/APNG), or go the other way and convert a video to a GIF with trim and frame-rate controls. If your GIF just needs to be smaller as-is, the GIF compressor will squeeze it without changing formats.