ok, first attempt using sketchup 8

[wilfy]

I'd use the box section.

Reasoning:

a) 1.7m of RSJ @ 19kg/m = 32kg, whereas 1.7m box section @ 7kg/m will weigh 11.9kg, thats nearly 2/3 less to move around...
b) RSJ will deflect .004mm in centre under its own weight plus 0.0004mm per kg load (i.e. negligible), Box section will deflect 0.018mm under its own weight, plus .004mm per kg load. So a 10Kg z-axis will deflect the rsj gantry .008mm and the box section gantry .058mm. In the scheme of things this is negligible.

so even though both are negligible as you put it.. you would still steer him clear of the easier/cheaper option of an rsj that needs little work over him welding box section and possibly leaving room for him to create errors...

or have i mis-understood... i'm watching this thread as the thought of rsj being an easy way compared to welding box section appeals to me