thought's pls on this radiator http://www.watercoolinguk.co.uk/p/Phobya-G-Changer-240-Bundle-with--2x-Phobya-G-Silent-12-1500rpm-Red-LED_18158.html
Either is fine, tubing size really comes down to personal preference nowadays; just make sure you get the same size fittings for everything, and be especially careful if they're compression fittings. I prefer 3/8" 5/8" because it's easier to route than 1/2" 3/4" and not as easily kinked as 3/8" 1/2".
the rad will be going in a cm690 II at the bottom with the outlets faceing away from the psu outlets faceing the front of the case...