Blenders and juicers are two very different beasts with two quite different purposes. Yes, they both process tasty fruit and veggies into a delicious, nutritious drink, but the way they do it and the end result are not the same at all.
A juicer separates the fiber from the rest of the fruit or vegetables. This leaves only the juice for you to drink. The remaining fiber pulp is discarded. This means that you can pack more fruit or veggies per cup, and you get all your vitamins and nutrients in the most easily digestible form.
A blender processes the whole fruit or vegetable, including the fiber. This makes a thicker drink (usually called a smoothie) which takes longer to digest. You still get all the nutrients, but they are released more slowly. This satisfies your appetite and leaves you to feel fuller for longer