Here are the stats:
Skim Milk Pros: less fat, less sodium, more potassium, more protein, less expensive
Almond Milk Pros: less calories, less carbs, 0 sugars, lactose free
If you are watching your calories, almond milk is a much better choice at 1/3 the calories!! (Just make sure to buy the unsweetened kind.)
Skim milk is way better in terms of protein. Along with carbs and fat, protein is a macronutrient, which means that your body needs relatively large amounts of it. Unlike carbs and fats though, your body doesn't store protein, so you need to eat it throughout the day to keep your body supplied.
If you are diabetic or on a low-carb diet for another reason, you would want to choose unsweetened almond milk. Almond milk also has 1 gram of fiber, which helps to keep you feeling full.
Skim milk is a better choice if you are cutting your overall fat intake.
Really, I could go either way on this - I will probably stick with the almond milk just because I need to keep my calories and carbs low. It's not going to add any extra protein, but that's okay because I only use it in my Shakeology or in protein shakes anyway. :)