From http://lowcarbdiets.about.com/od/nutrition/a/maltitol.htm
Maltitol is a sugar alcohol, an ingredient commonly used in low-carb or “sugar-free” products such as candy and nutrition bars. It is used so much because of its similarity to sugar in terms of taste, mouth feel, and interaction with other ingredients. Products which use maltitol and other sugar alcohols as sweeteners can be called “sugar free.” Although claims are often made that maltitol has little impact on blood sugar, this turns out not to be the case.
Maltitol is a carbohydrate. Although our bodies do not absorb all the calories in maltitol, this substance does provide us with 2 to 3 calories per gram, compared to the 4 calories per gram of sugar. (For what its worth, I have noticed that the claim of 2 calories per gram usually comes from literature provided by the manufacturer or the low calorie food industry whereas other analyses tend to be closer to 3 calories.) Since maltitol is a carbohydrate, and since it provides calories, you would expect it to impact blood glucose. You would be correct.
Maltitol Has a Relatively High Glycemic Index
In particular, maltitol syrup has a glycemic index of 52, which approaches that of table sugar at 60. The powdered form has a glycemic index of 36, which is still higher than most other sugar alcohols and all artificial sweeteners.

Then there's a bunch of good info on sugar alcohols at:
http://lowcarbdiets.about.com/od/whattoeat/a/sugaralcohols.htm

There you'll see that erythritol is a better choice; or combine xylitol and erythritol in your 'sugar' container. "Erythritol is the sugar alcohol (polyol) that has the least impact on blood sugar. Erythritol has almost zero calories, carbs, and glycemic index. The reason is a bit different that most sugar alcohols, which are only partially absorbed in the small intestine. Most (60-90%) of the erythritol is absorbed into the blood, but is then excreted in the urine. Because of this, erythritol tends to produce much less intestinal distress than other sugar alcohols."
Its a LOT harder to find and most likely you'll have to order it from a website.

Yes, pretty much cup for cup once you combine them. But that's just me, always wanting things simpler.
You'll always have to check on the sweetness factor cause you might use some stevia, or your cocoa powder might be higher quality (less other stuff in there) or who knows what else. I have to just keep viewing the whole new cooking thing as one big experiment. And it also depends on who I'm making it for -- DH or someone not on this eating plan - or me.