I use longer circs for blankets, etc. - things that require a lot of room for stitches. Makes it nice when you want to cap the ends and lay the WIP on the floor and assess my progress. I also use straights for small to med sized projects. I have some 7 inch chubby ones for scarves in a range of sizes and several bamboo and aluminum straights in scads of sizes in 10 inch and 14 inch.
I use 'em all!