Hah, I get to cite my new copy of McGee's "On Food and Cooking" for the first time! There are several things going on here (all of which can be found in the 2004 edition of McGee, most on page 50).
Firstly, as Nathan indicates in his answer, most of the liquid that is sold as buttermilk these days is in fact not "real" buttermilk, but so-called "cultured buttermilk", made from ordinary skim milk and fermented until acid and thick.
Secondly, even traditional buttermilk was somewhat sour, though less than cultured buttermilk. To find out why, let's examine the processes that were involved in making it. If you want to make butter, you start by separating milk into cream and whey. Since the 19th century, we typically do that with a centrifuge and it goes quite quickly, but before then, we would leave it to gravity. That would take a while, and the milk would start to ferment while it happened - especially in warmer environments. This fermenting is essentially lactic acid bacteria converting lactose into lactic acid, which sours the milk a little.
So in either case, you would have more lactic acid in butter + buttermilk than in butter + whole milk.
Best Answer
Milk, for the sake of simplicity, adds two things to a recipe: lactose and milk fat. Although in terms of flavor, it may make more subtle contributions to your food, chemically, these are the main things to worry about.
Lactose is the primary sugar present in milk. A cup of milk contains 12 grams of sugar, which is about the same as a tablespoon of white sugar. So using milk instead of water in your pancake recipe increases the sugar content significantly. Your pancakes will taste sweeter and brown faster; this could be an improvement or a detriment depending on your taste.
Milk fat, though absent in skim milk, contributes about 8 grams of fat per cup in whole milk. Since a tablespoon of butter contains 12 grams, this is significant increase in the total fat of your recipe. This could yield a richer taste, but too much fat and your pancakes might end up greasy.
So overall: milk won't add anything to your pancakes that they don't already have. But, increased sugar and fat could make them significantly sweeter and richer. Just watch out, and turn down the heat if they brown too quickly.
Carbonated water is simply carbon dioxide suspended in water. It is occasionally used in batters for aeration, especially in deep frying. But I think in pancakes the effect will be negligible due to all the activity from the baking powder. Further, carbonated water is slightly acidic, so it could throw off the pH balance of your batter.
If you really want fluffy pancakes, then separate the eggs, and whip the whites to soft peaks. You can incorporate a ton of air into an egg white, and if you gently stir it into the batter, the batter should end up quite fluffy.
Buttermilk, the fresh variety, is quite a thick product. Though buttermilk powder has all the flavor and acidity of the fresh stuff, it simply lacks the thick texture. You should try to buy a pint some time and see if it improves your pancakes.