In a technical sense, they're not similar at all.

ATACMS is a ground launched ballistic missile, so it follows a straight-forward parabolic trajectory: it climbs very high, goes very fast and then comes down on top of programmed GPS coordinates. Storm Shadow is a stealthy air launched cruise missile, it flies low at subsonic speeds and can manoeuver following a pre-progammed path (for example, to go around known air defense locations) and had advanced optics to locate the target.

Technologically Storm Shadow is way more advanced and it has a higher payload too. It also costs 4 times as much per missile, there are less of them and they can't carry a cluster bomb warhead because in Europe we decided not to make those weapons anymore.

Both would be very useful for Ukraine.