First, Squid is rare.
Second, Ice Golem practically has 9 HP, which is extremely hard to get rid of. Squids are weak and easily killable.
Third, Ice Golem has medium-high ATK. It can be used to attack as well.
As for the upgrade... For a deck with no upgrades, a single Freeze is required to get an Ice Golem, then that Ice Golem proceeds to convert other Water Golems into Ice Golems. For a fully upped deck, you can simply run six Ice Golems. This is how I want the difference between regular and upped to be, and personally I think it's about right.