For floats as a general rule I believe the rougher the water the more round and boyant the float should be. A thin float won't stay a float in fast turbulent water. Bright colored tops for big floats are also a plus cause they are much easier to see than dull colors, but maybe that is because I'm colorblind. Thin pencil shaped floats come into play when you are fishing slow water where sensitivity is king. There is virtually an endless amount of float styles that are avaliable, find some that you feel comfortable with, everybody has their own prefernce.