The F is for "full" (or sometimes FC for Full cut) and the number by itself is a skip tooth blade, which means that the teeth are set wider apart. The skip tooth blades are more suited to curly coats such as poodles and bichons, or terrier coats
I think you will find the 7f (or fc) will give a nice finish. The 5 blades give a bit longer cut but I find that depending on the kind of coat a cocker has it doesn';t always give as nice a fininsh