Cooldown runes Or ap runes

I love playing ap champions like , and . All these chamions are heavy on CD and are spamm heavy. With the cooldown mastery's and a mana regen item athene's or morrello you get up to 25% cooldown and 35% with blue. However you rarely get blue most of the game so i resort to three quints of CDR instead of the extra 15 ap. Wondering is this worth it?

Just asking to diamond+ no offence to anyone lower.