This is all my opinion, someone else may come along and prove me wrong or point out some stacking issues.
Ok, so with sleetstorm, it is a DC10 acrobatics to move at half speed, and then, unless you have blindsense or something, not being able to see halves that movement to 1/4 unless you make another DC 10 acrobatic check. If you fail either of the acrobatics checks you can't move.
Sleet Storm: Driving sleet blocks all sight (even darkvision) within it and causes the ground in the area to be icy. A creature can walk within or through the area of sleet at half normal speed with a DC 10 Acrobatics check. Failure means it can't move in that round, while failure by 5 or more means it falls (see the Acrobatics skill for details).
Darkness: For purposes of the following points, a blinded creature is one who simply can't see through the surrounding darkness... Blind creatures must make a DC 10 Acrobatics skill check to move faster than half speed. Creatures that fail this check fall prone. Blinded creatures can't run or charge.
Because it's dark, you would have to pass a check to move more than half speed.
Because it's icy, you must pass a check in order to move half speed.
There is no way to move more than half speed, thus the darkness is irrelevant for movement.
Cleric in sleet wants to move. He makes an Acrobatics check (at the +5 to DC). If he passes, he can move half speed. If he fails, he can't move at all this round (even if he spends another move action). If he fails by 5, he falls prone, and also can't move, though he could spend a move action to stand up.
One could argue that the Acrobatics check and movement penalties of Sleet Storm IS the effect of the ice. Or, you could view that as an additional penalty, and that the ground is also treated as walking on ice.
Ice Effects: Characters walking on ice must spend 2 squares of movement to enter a square covered by ice, and the DC for Acrobatics checks increases by +5.
In this case, lets say small cleric has a speed of 20', he makes his check, and can move at half speed: 10'. Each square is double movement (because of the ice) so he can move 5' per move action (barring diagonal weirdness).
With ashstorm you move at half speed, and then you have to make a DC10 acrobatics check not to have your movement reduced to 1/4. If you fail the check you can't move.
Ash Storm: Driving ash blocks all sight (even darkvision) within the spell's area of effect, and falling cinders cause the ground in the area to become difficult terrain.
Difficult Terrain: Difficult terrain, such as heavy undergrowth, broken ground, or steep stairs, hampers movement. Each square of difficult terrain counts as 2 squares of movement. Each diagonal move into a difficult terrain square counts as 3 squares. You can't run or charge across difficult terrain.
I see two separate effects: Each square costs double movement, and you must make a check to move more than half speed.
Heavy Plate Guy in ash wants to move without making a check: He moves at half speed, and each square takes double movement. His normal speed is 20' so he can move while effectively blinded at 10'. Each square is doubled so he can move one 5' square for each move action. (Not a 5' step!)
Spry Monk in ash wants to move faster, he's got good balance. His movement is 40' so he makes an Acrobatics check to move more than half speed. DC 10, his check is 24, so he's fine. He still has 40' movement but each square is double, so he can move 20' for each move action (Assuming he makes his Acrobatics check each time).
Foolish Bard in ash wants to do what the monk did. His movement is 30' and he makes an Acrobatics check. His result is 4, so he falls prone.
By the way, to me is seems like ashstorm is stronger unless you rule ice=double movement, while sleet storm is stronger if you do.
I think Ice might be better either way.
In Ash, you can effectively move 1/4 speed without making any checks. In Ice, you MUST make a check, thus risking falling, in order to move any distance at all.
Then if you add icy terrain to the spell's movement penalty, you've got no movement without a check for 1/4 speed (vs Ash's check for 1/2 speed or free 1/4 speed).
If you don't have a bonus to fire spells or fight cold-inclined creatures, then Sleet Storm seems to be better. Not all spells are equal, I don't see the need for a house rule on this.
This reminds me of a similar threat I made a while back called "Why Would anyone use Ash Storm over Sleet Storm?"