Thanks for the response guys,
I think I'll go 3x2 as its going to be a cube 60x60x60 or 61x61x61 haven't decided yet, ether way it will have 200+ liters of water so will be quite heavy.
Ismart120, did you get all of your wood cut to length at bunnings? That would of been a fair few pieces wouldn't it? Also what screws/nails are you using to hold it all together?
PS, How much is 3x2 worth these days?