You can download some tools to work this out;
80/20 have a nice free deflection calculator on their web site.
For steel and general beam deflections do a google for 'beamboy', a good, free program that I got the link for from another member here.
For material weights, prices and sizes try looking at onlinemetals.com
Deflection is a function of the load, support points, type of support, the materials properties and the beams length and cross section.
Each material has a modulus of elasticity, in English units measured in psi, that is a measure of how 'stiff' the material is. Aluminum is ~10,000,000 psi Steel is ~ 30,000,000.
The cross section makes a difference as material farther from the centre of the beam increases its stiffness. So a Tube with the same material cross section will be stiffer than a rod of the same cross sectional area.
Support makes a difference because if the ends of the beam are rigidly supported allowing no movement (cantilever type) the beam will be stiffer than if the beam is rested on supports (simple) that allow some degree of rotation at the support. It's difficult to get a real cantilever support in our machines so calculate using the simple support and that would be the worst case.
As an example a 3"x3" 80/20 (fractional 3030) 36" long with a centre point load of 250lbs would deflect 0.01". 36" and would weigh 3.93lbs per ft.
A 3"x3" Steel tube section with 0.25 wall thickness, 36" long with same point load 250lbs would deflect 0.002" and would weigh about 9.3lbs per ft.