Steve, You'll find a formula in Kreiges book, that gives a ratio of pole diameter to truss span of about 1/80 . So for under 2 meter length truss tube , 1" is fine. Wall thickness does little for stiffness but rapidly adds weight as you increase , which means larger mirror box to cope with higher side bearings, hence large rocker box etc., so I would use the 1.2mm wall.
Another advantage of using 1" tubes is that they will fit the new Moonlite aluminium split blocks, so using splits on the bottom and ball joints on the top makes for a very quick assembly both before and after you finish your scope.
I'm going to use this system on the 14" F4.75 binoculars I am building.
http://www.focuser.com/cgi-bin/dman....cgi&category=5
I'm also going to use their Delrin threaded tube inserts for the cage spacing tubes. I decided 20 X metal 1" threaded tube closures needed will add too much weight to my top end.