Flippers aren't the best in terms of building up strength, but they excel in fixing technique. The flippers will force you to correct your foot positioning, and how you move your legs. Great for butterfly, backstroke, and freestyle kicks.
Kickboards are the most common way of practicing your kicks. They are good for giving you something to hold on to, and letting you keep your head up to breathe. Kickboards will definitely give you a leg workout, so be prepared to feel the burn.