Defensively, the Ice Shield Tower has just over a a third-less life points of the Defense Tower, but gains the ability to castIce Shield on allied unit. The shield is capable of absorbing 880 damage at upgrade 3. Such a damage shield can more than double the damage that many first and even second era creatures can take before dying, which is clearly a good thing.
At a cost of 20 Power per cast of Ice Shield a third of the total life points of Defense Tower, it is easy to make a case for which card is better. However, defining better is very situational and circumstantial; if playing on defense with a lot of units to aid nearby and with time to do a bit of micromanagement, Ice Shield Tower may be the better choice. Otherwise, Defense Tower is a better choice to defend place without allied unit due to higher damage and HP.
One final note is that with upgrades, Defense Tower only upgrades its damage, while Ice Shield Tower upgrades damage as well as the damage shield, making the upgraded Ice Shield Tower an even more compelling choice over upgraded Defense Tower.
Note: One unit can have only one Ice Shield on it. Damage absorbing effects (e.g. Ward of the North) do NOT reduce the incoming damage on shielded units as long as the unit has the shield.