Electricaly they are susposed to be the same as I understand it. 8ft will transmitt and recieve just a bit better. Line of sight method it stands a bit higher in the air. SImular thinking to having rabbit ears on top of your tv versus on top of your house, Which would get better recption> A=On top of the house.
where are you planning to use it at? big water or river? I have an 8 ft on my boat and its just in the way most of the time. For most things I would say a 4ft would be plenty.