I've been thinking about this topic much because I don't know which of those two to use as primary CC-shout. Both have pros and cons, I'll list them here:

Unrelenting Force - Pros:
-Works on everything, you can throw around giants and at least stagger and interrupt dragons
-Has a lower cooldown than Ice Form (45sec on lvl 3)
-Can be a killer when there are cliffs etc around ^^
-Sending your enemies flying is always fun... xP

Cons:
-Enemies get up quickly and start attacking you again
-Does no (or at least almost no) damage itself

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Ice Form - Pros:
-Takes enemies out of combat for a long time (20-30sec)
-Does decent frost damage over time, at least to lower minions, which means you can take out the strong foes and kill the weak ones in one shout
-This is not really a "pro" but I think the shout itself sounds cooler than Unrelenting Force... xD

Cons:
-Has a much higher cooldown than Unrelenting Force (60sec on the first lvl, 90sec on the second and 120sec on the third)
-Almost useless against Dragons as it will only deal minor frost damage to them



So guys, debate and help me decide which one is better! I'd like to hear your opinion because I cannot really decide myself

ice form works on giants

Tried it immediately after you answered and you are right, I'll correct it asap. Thanks!

Ice form hands down, you can freeze half an army and pick them of one by one

ice form can be used to pickpocket items which have a 0% chance to steal. (only tested on vex and delvin)

For Dragons Ice Form is crap use Fire Breath on Dragons. All Dragons are weak to fire.

You're not serious are you? The only dragons that have a 25% weakness to fire are frost dragons. And I'm not talking about a way to fight dragons here, I'm talking about the CC-effectiveness of the two shouts mentioned in the OP.

I use both these shouts the most. It depends on what kind of foes I'm facing as to which one I'll use and I mostly use them on groups of combatants. If it looks to be a tough fight I go with ice form to completely take a foe or foes out of the equation. If it's easier enemies I use unrelenting force because it's just plain awesome. And I've had unrelenting force kill weaker opponents.

paralyze spell in alteration works as well.

The way you phrase the question I can't help but think the answer is obvious.

Unrelenting force - enemies get up quickly and start attacking again

Iceform - enemies frozen for a long time

Just wanted to notify you that the winner has been decided:

It's definitely Ice Form as it freezes you opponents for about 60 seconds (!!!) on level 3. A friend bought the strategy guide and I looked it up there.

Though I love unrelenting force I would go with ice form.

Used Ice Form against six bandits charging me in the first Thieves Guild mission. Their momentum, combined with the slippery ice they were encased in, led five of them to keep on travelling right into the river, from where they could not escape, and I picked them off with my bow.

Unrelenting Force is good, but needs open spaces to be viable as a crowd control option. Launching someone into a wall for minor damage doesn't do it for me. Freezing someone so I can summon a Flame Atronach and poison my weapon does.