If you are worried about the capasity of the 21 then it is too small for you, and you must be cutting big things with it. The 30 is equel in performance to the 21 and all control is from the front once you have set the tension at the back. be aware that they are built like tanks and are very heavy so need to be static. If you watch Steves vid on the 21, then you have it, as the 30 is just a taller brother Iggy.