Cake recipes: sour cream vs. milk, butter vs. oil, etc.

I agree with some of the other bakers here: oil makes cakes more "shelf stable." Meaning, cakes will hold up at ambient temperatures better and stale more slowly, than ones made exclusively with animal fat dairy.
On a side note, I have 100% replaced all my recipes needing sour cream or creme fraiche with Labneh/Greek yogurt. It's usually less expensive, less fatty, less liquidy and more acidic. Acidic dairy tends to make a cake more moist, but also add to the flavor's complexity.
A cake made moist by butter and eggs alone is good too, but the same cake with buttermilk or yogurt or sour cream tends to be more interesting.

Over the years, I have found that oil (versus butter!) in white cakes makes them infinitely lighter, more moist, able to last well for days and have a much better crumb.

Sorry, here is the link to the recipe I mentioned:
http://www.foodnetwork.com/recipes/gale-gand/chocolate-layer-cake-recipe/index.html

The second link is to a second-hand source for the book. Well worth every penny of its $3.98 price.

Sour cream-based cakes will be a bit more dense than those prepared with milk. One of my favorite cakes comes from Gale Gand's book Butter Flour Sugar Eggs. You can take a look at the recipe here: http://www.betterworldbooks.com/butter-sugar-flour-eggs-whimsical-irresistible-desserts-id-9780609604205.aspx

The book is, most unfortunately, out of print, but you can pick up a used copy here, which I would highly recommend. It's a treasure, and I refer to it often.
http://www.betterworldbooks.com/butter-sugar-flour-eggs-whimsical-irresistible-desserts-id-9780609604205.aspx

This sour-cream cake, though is not remotely dense because it is leavened with a whopping full tablespoon of baking soda. Baking soda is used to leaven acidic mixtures. The acids here are present in the sour cream, cocoa powder, and coffee.

You can replace part of the milk in any recipe with sour cream by adding baking soda in the ratio of 1/2 teaspoon for every 8 ounces of acidic ingredient. I probably wouldn't replace all of the milk with sour cream, but rather perhaps half and half, whisking the milk into the sour cream to loosen it up. The higher fat content of sour cream will tenderize the cake a bit more.

As for butter vs. oil, there was a thread here recently on the subject. It's worth looking over for some good information. http://food52.com/hotline/18769-carrot-cake-vegetable-oil-or-butter

Happy baking!

My experience has been that sour cream adds a deeper flavor than milk and also makes for a richer denser cake. Butter adds better flavor but oil makes a cake more reliably moist. It all depends on the recipe too!