Cloyster moveset

Frost Breath hits faster therefore it allows dodge/weave capabilities and will dish out more DPS than Ice Shard because it's a slower attack.

If you aren't dodging, they are actually pretty close because though it's slower, Ice Shard generates more energy.

For gym use, however, I prefer Frost Breath in practice for its speed.

Frost Breath is better on the attack, Ice Shard rules on defense, exactly as you say. So it depends on how you want to use your Cloyster. With all the dragons entering the game in Gen3, many of whom are doubly weak to Ice, an attacking Cloyster will be a very valuable asset. And a defending Cloyster will be a thorn in the side of everyone who wants to sweep a gym with a dragon.

Mine has Ice Shard/Avalanche and it is fun...it is a tank.