Frozen Mallet is much more simple so I will start with that.
650 Health
Very reliable item for tanks. Health is generally one of the most core features for any tanks because even armor and magic resistance is useless without it. Enemies can pretty much always deal more damage than than you can mitigate. So even if you can significantly reduce the amount of damage you take, it means nothing if the enemy can quickly burn through your messily health pool.
40 attack damage
Makes you into more of a bruiser. Works great for AD scaling casters or auto attackers. Simple, more damage stats, more damage dealt.
Icy Passive
This is where the difference really comes from in my opinion. You can really only take advantage of this item if you regularly auto attack your enemy, which can actually be more difficult for melee champions. I would really suggest this item for marksmen who are good at kiting or keeping enemies from running away so they can continue to harass, or fighters who can naturally stick to their enemy, but want that extra slow to ensure that it is more difficult to escape.
Iceborn Gauntlet is a bit more tricky.
65 Armor
As said before, armor is a good defensive stat for reducing damage, but not as effective without health. I would expect to see this more in conjunction with items that provide health or champions with built in health. Also works fairly well on champions who can either scale on armor, or expect to receive the occasional poke, but don't expect a full on assault.
20% Cooldown Reduction
Quite simply, this works well on casters. This single item gives almost half the amount of CDR possible.
500 Mana
Great amount of mana for champions who frequently run into mana issues. Also synergies well for casters.
Spellblade Passive
I am about to say the magic word that I think will finally answer your overall question. .. Casters. This item works great with champions who naturally have low cooldowns, which are lowered even more by the CDR provided, sustained by the mana. The Armor is really more of a bonus, and likely simply a side effect of the build path, but it helps. The closer you can get your cooldowns, or cycle your abilities to 1.5 seconds, the more useful this item becomes. Also, keep in mind that this effect grows larger in radius based on how much armor you have. So stacking armor can improve it's effectiveness (although I rarely go for this). And most important, the bonus damage added to your next auto attack from spellblade is based on 125% of your BASE AD so champions who have high base AD are even better with this.
So in summary
Frozen Mallet is better for auto attacking marksmen and fighters.
Iceborn Gauntlet is good for casters.
I hope this helps. Sorry if I over explained anything.