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.
That goes for any unexploded ordnance, we are still cleaning up regular unexploded shells from World War 1 more than 100 years after the fact and every now and then it still claims a victim.
It sucks, but you have to offset that against the benefit. The longer the Russians occupy parts of Ukraine, the more atrocities they are able to commit against civilians (cf. Bucha, Irpin, Izium, Kherson,...). Also when people talk about the civilian casualties, they always forget that the bulk of the Ukrainian soldiers were civilians just over a year ago, and they would love nothing more than to return to a peaceful civilian life. Their lives are valuable as well and should be protected too.
If cluster munitions helps them to get rid of the Russians faster and with a lot less casualties, it is a trade off we should make.