I've always played that ability damage and drain stack. The main difference is that ability damage is temporary and applies a penalty to stat rather than changing it like drain does.
Diseases, poisons, spells, and other abilities can all deal damage directly to your ability scores. This damage does not actually reduce an ability, but it does apply a penalty to the skills and statistics that are based on that ability.
For every 2 points of damage you take to a single ability, apply a –1 penalty to skills and statistics listed with the relevant ability. If the amount of ability damage you have taken equals or exceeds your ability score, you immediately fall unconscious until the damage is less than your ability score. (cont...)
Ability Drain
Ability drain actually reduces the relevant ability score. Modify all skills and statistics related to that ability. This might cause you to lose skill points, hit points, and other bonuses. Ability drain can be healed through the use of spells such as restoration.
But this seems to contradict that notion.
Ability Damage and Drain (Ex or Su)
Some attacks or special abilities cause ability damage or drain, reducing the designated ability score by the listed amount. Ability damage can be healed naturally. Ability drain is permanent and can only be restored through magic.
And then bleed seems to confirm it saying they are essentially the same.
Bleed
A creature that is taking bleed damage takes the listed amount of damage at the beginning of its turn. Bleeding can be stopped by a DC 15 Heal check or through the application of any spell that cures hit point damage (even if the bleed is ability damage). Some bleed effects cause ability damage or even ability drain. Bleed effects do not stack with each other unless they deal different kinds of damage. When two or more bleed effects deal the same kind of damage, take the worse effect. In this case, ability drain is worse than ability damage.
Putting all this info together I think I came up with suitable comparison. Damage vs nonlethal damage. If I bleed for damage and nonlethal, then only the damage will be applied (perhaps some NLD if more than damage?). Would that be the same for ability damage and drain. It says take the worse effect which suggests dropping all the lesser effect even if the effect is more for the lesser.
So, if I take 1d4 bleed nonlethal and 1 bleed (lethal), do I ignore the nonlethal bleed or roll it and apply the nonlethal damage that's above the lethal? That seems like the right thing to do.
Extra: I was trying to think of the effective difference for damage and drain. Damage applies specific penalties base on stat, but won't affect things like extra spells or qualifying for feats, but drain will. Damage is also applied only every 2 points. So, having a 14 dex and 1 dex damage won't reduce your AC, but drain will.
I would also apply ability damage and drain as if they stacked - start with a Constitution of 14 and suffer 2 points of ability damage and 2 of ability drain, The drain effectively reduces the Con to 12 and with a further -1 to Con modifiers due to the 2 points of ability damage.