Regarding the overall question, even putting aside immunities it's hard to argue against Cold simply because it has such a powerful arsenal and Fire doesn't.
I don't really love displacement. If you are a warforged, I like having Repair Serious for an echoes-able cure. Mass Circle Against Evil is great if you couldn't fit Protection from Evil in level 1.
Originally Posted by rikkitikkitarvi
Not very. It's Illusion school and has two saves. However, a sneaky benefit is that an enhancement in the Draconic Incarnation ED offers +3 DC to fear-based attacks. I still wouldn't recommend it, but I would definitely take it over Stoneskin. Ablative damage prevention has been dwarfed by escalating incoming damage, it's not worth the time it takes to cast. Ice Storm is another very versatile choice.
At Lvl 4
Water: Acid Rain, Dimension Door, StoneSkin, Phantasmal Killer?
Earth: Acid Rain, F.Wall, Dimension Door, Stone Skin?
How useful is Phantasmal Killer in the upper lvls?
The trouble is you have to make a check against the enemy caster level, and especially in epic content enemy caster levels are through the roof. It's the same reason PC Spell Resistance is so pointless. You should always have both Eladar's Electric Surge and Niac's Biting Cold, even on an Earth or Fire savant respectively. They're just too powerful and you really don't lose that much from the caster level penalties. Regarding Protection from Elements please see Stoneskin.
Water: Protection from Elements, Cone of Cold, Cyclonic Blast, Eladar's Electic
Earth: Protection from Elements, Cone of Cold, Cloud Kill, Break Enchantment
Is break Enchantment useful?
Disintegrate is good but not terribly reliable, and especially as a savant I think you'll have enough direct damage buttons to keep on cooldown without it. Necrotic Ray's raw damage isn't impressive, but it has two interesting niche applications: targets that take negative but no elemental damage (such as Shroud portals), targets you want to negative level but can't bypass SR. Level 6 is a tough one, no two ways about it.
LVL6: This is where I really have an issue with what to take.
Water: Necrotic Ray, Otilukes Freezing Sphere, Disintegrate, G. Heroism (Can only take 3)
Earth: Necrotic Ray, Otilukes Freezing Sphere, Disintegrate, G. Heroism (Can only take 3)
Never used Necrotic Ray, but the damage and range look good. Is it really as useful as it looks?