Cooldown Quints worth it?

I love cool down reduction many a fight im 0.5 seconds off killing an enemy with an ability but with cool down reduction this saves me. unless im an adc or usually tanks i always have 40% cool down reduction, the max you can get. With this im in a habit of buying CDR boots over penetration boots. This puts me in a great setback to damage i feel 15 magic pen or 15% cdr boots is tough to decide. What is better 15% CDR at lvl 18 and magic pen boots or extra starting 15 ap and cdr boots?

Sorry if i have bad grammar (lazy)