My .02 Comparison
Fireball - avg damage at 7th level (24.5) with a save for half
Ice Storm- avg damage at 7th level (18.5) without a save
Both have a chance for spell resistance. At this level, Ice Storm is superior damage wise, since there is some chance of it doing less damage (if everyone in the fireball saves) and it does magical physical damage (very rare). A saved fireball will only do 12.25 and while there are characters who have poor saves there are also characters who will save for 0 damage (with evasion). So basically, imo, Ice Storm is better for damage since it is guaranteed to always do 75% of fireball, and sometimes it does 125% or even damage where none would be inflicted.
Keep in mind that the elemental resistances balance out, more or less. Cold and Fire are just about equally prevalant in the MM, though there are less Cold spells in the PHB.
Ice Storm also lasts a full round, so it creates an area you don't want to pass through (good for creating choke points) and even if they do want to pass through, movement is halved.
The comparison for damage worsens as Fireball and Ice Storm level, as seen below:
Fireball - avg damage at 10th level (35) with save for half
Ice Storm - avg damage at 10th level (18.5) without a save
The comparison is now Ice Storm always doing 50% of fireball, or doing the same damage. Ice Storm still has slight tactical advantage, but at this point the spell level discussion comes into play. At 7th level Ice Storm was a superior choice because Ice Storm was also a level higher than fireball, hence it SHOULD have better (or more consistent) damage. At 10th level fireball seems to outstrip it doing the same (or better, considering enemies' reflex saves) damage for part of a round (vice full round).
This discussion becomes even more interesting when you bring up metamagic.
For a 6th level spell I present the following:
Empowered Fireball - avg damage at 10th level (52.5) Reflex half
Extended Ice Storm - avg damage at 10th level (18.5) for 2 rounds
While it may appear tilted towards the fireball at first, we must account for the halves like we did earlier. Remember, assuming we can keep someone in the Ice Storm for 2 rounds, we are doing double damage, at the cost of 1 spell level. So the Ice Storm does about 30% more damage if it hits for both rounds and the fireball is saved against or 40% less damage if the fireball is not saved against and the Ice Storm hits for both rounds. If the fireball is not saved against and the Ice Storm only hits for 1 round the fireball is roughly doing 200% more damage, at the same level.
This is where we examine the spells again. Fireball is a straight-up damage spell, it can also catch things on fire, but other than that all its good for is hurting. Ice Storm can possibly prevent a retreat or cause excellent damage against rogues (of course rogues are also the most likely to get out of the way). Ice Storm is also very good against wizards (esp rog/wiz) because of the lack of save. While its true that the fireball is seeming to dominate, remember that when characters can cast 6th level spells they can also pick up rings of evasion (and those low level characters with evasion are sporting Imp Evasion, half damage all the time for fireball).
Ice Storm is also accessible to druids (although druids get Flame Strike at 4th, which presents itself as nearly well against Ice Storm as fireball does, with some differences (smaller radius, half divine damage)).
Overall I think Ice Storm is just a touch too weak. I would improve the cold damage to 3d6, making it a full 6d6. This strengthens it at all comparison levels, making it a better buy early on (for a higher spell level) and making it slightly more competitive at higher levels. I should note that while Extended Ice Storms are excellent against Wizards, Sorcerors, Bards, and even Rangers (if their hp is low) it sucks against Monks, whose high movement ensure that you will only nail him for 2 rounds in an enclosed area.
For some situations, the lack of evidence (besides damage done) is another thing in favor of Ice Storm. Cast fireball in a bookstore and you have a new enemy (the store istelf), cast Ice Storm and you merely have some broken merchandise (still a problem, but not nearly as bad).
Widened Ice Storms are quite good also, the larger area makes it more unlikely for a character to get out. An 8th level Widened Extended Ice Storm does (under my revision to 6d6) 21 damage for 2 rounds with no save in an 80ft radius (meaning a medium-size creature at its epicenter with 30 ft movement (moving at half-speed) could not escape with a double move. As it lasts 2 full rounds, wailing on them with more area spells is easy, it keeps your tight bunch (which usually spreads out after 1 successful area spell), so you can follow up with your Maximized Empowered Fireball (which avgs 77.5 damage) and they still get pelted with the Ice that round!
Technik
