overall space frame torsional strength as members with increased tensile strength.
Young's modulas for all steels common to us are similar as to be considered the same, only the yields change.
So if you have lengths of 1010 through to 1020 and stand in the middle, they will deflect the same amount just the 1010 will eventually bend (as in stay bent/kinked) before the 1020 - but the levels of failure/force we are talking about would see your body ripped apart so again, no need to think about it, just grab what looks like reasonable quality at your local steel supplier and just be careful you don't get hot rolled re-cycled steel and that's only because it's a pain in the ass to weld with..
The differences though for our level of building
isn't worth a minutes thought and chrome moly is a waste of time and money.
Material size choice is the most important, I dummy'ed up a couple of DeDion tubes recently, one from 2 lengths of 1.65"x.080" box and one from 3"x.080" tube and I can stand on the double 1.65" and jump up and down in the middle and get flex (actually feels soft to stand on) whereas the 3" tube doesn't budge at all, not a bit.