There are 2 main types of juicers- centrifugal and masticating.
Centrifugal juicers use high speed to extract juice, as noisy as a blender and are somewhat lower in efficiency of extraction, and are usually cheaper and hence popular. They are good for juicing fruits, and not so good for greens.
Masticating or the cold press type juicers use low speed to extract, very quiet, more efficient and more expensive than centrifugal type juicers. The Omega juicers belong to this category. They are best suited for greens. Since they generate no heat because of the low speed, most enzymes from the juice are intact and there is very little degradation of nutrients due to oxidation.
My recommendation if you want to buy a juicer for green juice: shop for an Omega juicer and wait for it to go on sale!