My problem with Cloud of daggers is that is becomes a simple minion killer once you pass a few levels. It'll only deal Wis mod more damage than the other at will. At level 7+, it'll be 3-5 more damage and auto-minion kill. I could use SBurst for area, RoF for 1 turn slow, MM for range, IA for -2 to attacks etc.
Also take the feats into consideration
Ray of Frost is great with the 2 cold feat combo (wintertouch+lasting frost). If you hit you get +2 to attack and +5 damage for cold attacks. It'll be great when all your enc and dailies are done. And not all brutes high high Forts. And those speedy skirmishers and lurker have terrible Fort.
Thunderwave with Arcane Reach and Resounding Thunder gains some range. Add solid sound for defense help.
Illusionary Foes is psychic right? Psychic lock = -4 to attack.
Yup, feats help. Inescapable Force and Solid Sound can help Cloud of Daggers.
But with regard to the 3 (core) single target At Will Wizard powers, nothing really changes that much at high level. They still have the same range, they still do about the same amount of damage.
As for Fort, sure, some creatures have low Fort. Some. It's ~18% with better Fort than Reflex (give or take, I have not calculated it for all creatures).
That's less than 1 creature in 5 with better Fort than Reflex. That means that the +2 to hit you are talking about for WinterTouch (which does not occur until turn #2, the creature has to be hit with cold on turn #1 in order for WinterTouch to affect it on turn #2) merely negates most of the difference between Fort and Reflex for 4 out of 5 creatures (and the difference for these 4 out of 5 creatures is closer to 3 than 2 due to the 1 in 5 creatures which are more susceptible to Fort bringing down the average).
WinterTouch is an entire feat whose purpose is to put cold spells on par with non-cold spells with regard to "to hit", but only if the Wizard takes the Lasting Frost feat as well. Without Lasting Frost, WinterTouch totally sucks. WinterTouch is also worthless for the first cold attack against each creature. It doesn't kick in until attack #2.
So sure. Lasting Frost will give +5 damage. Compared to +Wis to damage hit or miss for 10 character levels of Cloud of Daggers. Two feats, just to get Ray of Frost on par with Cloud of Daggers.
That sounds like a lot of feats just to get in the same ballpark.
Let's take a simple example.
50% chance to hit. 11th level Wizard with +2 implement, 21 Int and 15 Wis (i.e. the Wizard never bumped up his 1st level 14 Wis, this favors your POV). We'll ignore the 1+ better Fort 1 in 5 creatures and consider it a wash with the 3+ better Reflex 3 in 5 creatures (and the fact that WinterTouch does not give a +2 bonus on round one) for simplicity.
Cloud of Daggers does 50% x 3.5+7 + 100% x 2 = 7.25
Ray of Frost does 50% x 3.5+12 = 7.75
With a gimped Wisdom Wizard with zero feats, Ray of Frost with two feats does slightly more damage. One would expect two feats to do more than this.
Up the Wizard's Wisdom to 19 (which at level 11, many PC Orb Wizards will have 19 or even 21) and it becomes 9.25 CoD damage vs. 7.75 RoF. And, there are other ways to get Combat Advantage which puts Ray of Frost back into the Fort saves sucks category again (60% x 3.5+7 + 100% x 4 = 10.3 vs. 50% x 3.5+12 = 7.75).
Ray of Frost is the weakest of the At Will single attack powers and needs feats to help it out. The Slow is practically irrelevant to the conversation. It hardly affects combat at all compared to killing a minion nearly every time CoD is used against one.
All in all, Cloud of Daggers is better for most applications than Ray of Frost and this is especially true for levels 1 through 10 (and still better than Ray of Frost for levels 11+).
Note: Sure, Slow can help in conjunction with other powers like ones that create Difficult Terrain. But, Slow for one creature rarely does anything when most powerful creatures have ranged attacks.