Preference and what/where your fishing. Most my fishing is from shore either smaller pan fish up to trout or catfish. If it's smaller ponds or technical areas as well as smaller fish i use 6'6". If its a big pond or in a boat where i need to do deep casts, while going for bigger trout or catfish then it's 7'. I do flyrod and thats a bigger pole for a different purpose, then there's surf fishing where i bring out big betty my 11', but thats a total different fishing.
My suggestion, buy either a pole with 2 tips so you can switch out length. Otherwise 2 poles, one in each size. We are fisherman and love gear.