Um, on is 3/8" and one is 1/2"... The 1/2" will have a better flow rate, but that isn't the be all and end all in WCing, it will depend on the other components in the system.
is that watercooling system any good? DangerDen RBX CPU Block: TDX Intel Socket 478 - 1/2" ID DangerDen MAZE4 GPU nVidia/ATI Block: DangerDen MAZE4 GPU - 1/2" ID Hydor L35 Pump Tygon Anti-Kink 1/2" ID Tubing Danger Den Black Ice Radiator: Xtreme II 120mm Dual Fan (Black) Innovatek innovaProtect Fluid Arctic Silver 5
I guess thios is your first step into watercooling? Check these kits, all good choices depending on your budget, and you can customise them with alternate parts just by emailing them. http://www.coolercases.co.uk/water_cooling_dangerden_kits.htm
Thats just quite a (I hate the word) newbie question. Not that its a bad thing, we were all there at some point Oh you didnt list a res in your parts lists either...
Id say you do need a res if you will be running the system long term, its hell to top-up a system with out one, unless you have a fill/bleed pipe but thats the ugly option.