Juicing and blending are two very different things. Many people want to know which is better and why. The answer is that they are both great… but for different reasons.
Using a juicer eliminates all the fiber from the fruits and vegetables. Fiber is good for you but depending on your diet and personal health goals you may be in a position to benefit from its removal. Removing the fiber from your fruits and vegetables will allow you to gain all of their micro-nutrients quickly and without the bulk of the entire food. This method works well for those who are trying to add nutrient-density to a diet already rich in fiber. However, if you are trying to increase your fruit and vegetable consumption blending may be a better option to ensure you reap the benefits of the additional fiber.
Also, some people have a hard time digesting raw fruits and vegetables. People with very sensitive digestive systems often find it a relief to be able to drink the equivalent of several cups of vegetables without irritating their digestive systems.
Juicing does, however, have the potential to cause spike in blood sugar level. Without the fiber to slow down digestion, your blood sugar can rise very rapidly. For example, a juice made entirely from apples or carrots can give you a sugar rush which may leave you light headed. In order to mitigate this you can mix bitter fruits and vegetables in with the sweeter ones. A properly made juice should be high in celery, leafy greens, cucumbers, lemon, or other bitter fruits and vegetables to outweigh the high fructose content of the sweeter types (which contain simple carbohydrate that is healthy and perfectly suitable in reasonable amounts). Since blending retains the fiber found in your smoothie ingredients, rapid digestion and blood sugar spike are less of an issue.
Blending is also much more filling, and thus more impactful as a meal replacement. Using juice to replace your meals may be good as a temporary measure (e.g. for a cleanse), but not as a part of your regular routine. If your goal is to grab a quick breakfast on your way out the door, the blender is a better tool for you to use.
All in all, both blending and juicing are excellent ways to get nutrient-rich foods into your diet. That said, both have a specific purpose and thus likely have a role in your kitchen. The choice is not to blend or to juice, but when to employ each appropriately.