If you ask me, green juices are great for quick nutrient absorption, whether it's first thing in the morning or as an aid to fasting and detoxification. Green smoothies are a great and convenient meal replacement, full of nutrients and hardy fiber.
In my own experience, I have found that incorporating both into my diet is most effective. I'll usually start my morning off with a low-glycemic green juice (see my Juice of Radiance recipe) followed by a green smoothie. This way I've hydrated and energized my body with a green juice, and then satiated my appetite with a green smoothie!
But I will say that my personal preference is definitely green juice! So much so, that I'll typically have another green juice later in the day... after my evening workout.
I love how refreshing and energizing green juice makes me feel, not to mention how hydrating it is for the skin. I would even go as far as to say that green juice is truly nature's fountain of youth and secret to a natural, radiant glow!
But again, both are wonderful and definitely worth incorporating into your diet! So play around to find your balance, and enjoy them both!