They are both bad, because they both mean you are doing things "on" the object, or "with" the object, instead of encapsulation of the logic "in" your object.
Just call a method on the object (for example: "Open"), for all normal drinks you can implement it in the base class, for CocaCola you can override this class and implement "return 'tssssssss'" (the sound of a coke opening).
Better yet, don't use inheritance, use a behavioral design pattern.

I cover some of these lessons life has thought me in this blog post: "http://janvanderhaegen.wordpress.com/2011/11/04/coding-is-optional-a-random-walk-down-oo-road-less-is-more/"

I disagree. I think "as" should only be used when you have a default value.
The problem with as is
1. you can't use it on structs, so if you use "as" you'll have to occasionally use "is" anyhow. so you code becomes inconsistent.
2. Correct exception thrown. Sometimes you won't bother with the is. a (Foo)Bar throws an invalid cast. Using as will throw null reference.
3. Minor, but better scope. declaring the varable inside the if limits the scope more.

As should only be used if you have a default value. In this case you can say
var myString = myObject as string ?? "Some default message";