Mike, stiffness and damping are two different things. Neither aluminum alloy nor steel has any appreciable damping qualities. They both make nearly perfect springs (or tuning forks, if you prefer). That is, they store energy when bent within their elastic limit, and give just about 100% of it back when the load is released. OTOH, damping devices (like car shock absorbers) take the energy, turn it into heat and dump it overboard. I.e., they don't give it back.
Rotor vibes are very low in frequency; obviously a 1/rev is six-ish cycles per second, and a 2/rev is twice that. Well below the lowest frequency we can hear, and not likely to resonate in any mast that is strong enough to resist the bending loads we talked about. You can find the resonant frequency of a mast by clamping it at one end and giving it a whack, just like a big tuning fork. I haven't tried it recently, but I believe you'll get an audible note -- meaning its resonant frequency is many times higher than your rotor vibes.
Once you pick a tubing size and wall, you can check its spring constant to see how soft it is in bending, compared to a standard mast. Obtain one mast of each type. Clamp each one in a horizontal position, restrained at one end as a cantilever beam. Hang a known weight from the other end and measure how far it bends.
I haven't tried it, but I suspect that a 4130 mast with appropriate wall thickness and the more usual 2x2 x 1/8 6061-T6 will not be that different. That is, both will flex and isolate the rotor head a bit -- but remember that Bensen's flexing-mast system has never been ideal. It's simple and tolerable in small gyros, but even they can be made smoother with a more elaborate "soft mast" suspension or slider head. Such a suspension becomes just about mandatory with the 3/4 ton, 2-place flying tanks that many people seem to like.