depends alot on your application and how it will get used. these guys are all right on in what they're saying - heres a few other things to consider.
on the average cage ill use a minimum of two kinds of tubing and at least 3 or 4 sizes for the following reasons. in general members have a variety of loads placed on them and there all a little different depending where they are in the structure. a good example is an interior member, (meaning interior to the volume of the cage, not nessicerily in the vehical) that is in tension with occasional compression would only need to be 1/2 the size and strength of the exterior bars. ive used as little as 1" od .060 mild hrew in those locations without failure. BUT, you have to use it in the right spot. on the other hand, DOM, for its cost, has superior impact resistence so its a natural for sliders and lower members where gravity and your rigs weight is headed most of the time.
my point is that 060 1" inch hrew is a fraction of the cost of 2" 120 DOM or even the dreaded high carbon stuff. so in tuning your design even a little, you end up with a weight and cash-outa-yer-pocket effecientcy by putting the beef only where you need it.
theres alot more to talk about but thats the basic idea - good luck, and post'em as you go