This has been debated many times but here are my reasons-
8mm because-
Less pressure required in system thus less strain on everything including connectors etc.
Can run pump much slower as there is less resistance meaning less power required from battery, less strain on pump. (mine will run all day from cig' lighter socket in van)
Flow more reliable & controllable
Cleantech used to do ( & probably still do ) superb 8mm ID hose. It wasn't much bigger OD than 6mm hose. You could get 100m on a 50m hozelock reel.

Most suppliers 8mm hose has a bigger OD than the Cleantech one.