G 1/8 is the pipe size and not the internal diameter of the tubing. The outgoing connectors can utilize 6mm (1/4"), 8mm (5/16"), or 10mm (3/8") fittings. However the kit comes with 6mm ID tubing.
The Aquaduct 240 and Aquaduct 240 Pro both have a 240 size radiator in it. The Aquaduct 360 XT has a 360 size radiator in it. The 240 sized Aquaduct's will be fine, but if you want some room for additional blocks you should consider the 360 XT.
If you want to see the 360 XT opened up, you can
go to this thread.