Which skill is better, Iceberg or Glacies?

Depends. One has a shorter cooldown, but lower damage while the other has a longer cooldown, but higher damage.

If you have a skill like Heavy Blade, or Quicked Pulse, then Glacies is a more...viable choice. If not, then Iceberg is probably the best choice for it's smaller cooldown.

There's also things like Quick Riposte, so... :l

It depends on the character and their build...

For a high res nuke without a cooldown accelerator, Iceberg is superior.
But for a high res wall or a nuke with a cooldown accelerator Glacies is superior.
High res duelists can go either way, depending on their other skills and specialties.

Depends on the character's stats and other skills.
If you have slaying weapons, steady breath or some other form of cd reduction, glacies is better, otherwise go with iceberg

It depends on what character you are going to use it, for example felicia has to use glacies since she has such low atk, also slow character with decent attack use glacies very well, take Julia she can't oneshot but if you give her QR, normally in a fight she will kill the enemy and have glacies ready to nuke the next enemy. Iceberg instead works well on fast and hard hitting characters for the times they can't kill the enemy the special will help them. Obviously this also applies on Bonfire and Ignis for character with good phisical bulk

Usually Iceberg, unless you run Quickened Pulse or a “-1 to special cooldown” weapon(or Steady Breath/a unit with Infantry Pulse). In that case Glacies would have the same cooldown as Iceberg and hit harder of course. Some examples are Lukas and Gwendolyn with Ignis. They tank two hits, and retaliate twice thanks to Quick Riposte with a 30-35-40 damage special(depends on the build).

So, usually Iceberg for people with high res that don’t have a Killer weapon effect. Then Glacies for people with Killer weapons/some form of cooldown reduction in their build or people that usually rely on their special as their main source of damage(Henry). There are some exceptions(for example Joshua doesn’t always get doubled and if he doubles he won’t proc Glacies, this doesn’t happen with Iceberg). There are also other units that don’t have a Killer effect weapon but run things like Vantage and try to find the best opportunity to set up their special so it procs in the Vantage counterattack.