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I'm looking at page 3019 of the McMaster-Carr catalog. We're looking for a 10-pitch lead screw that offers sufficient strength, minimizes weight, and that is as cheap as possible.

I'm currently between a "Grade B7 Alloy Steel 3/8 inch - 10 pitch" lead screw or a "zinc-plated steel 1/2 inch - 10 pitch" lead screw (in a 6 foot length, they both come to below $30). The B7 alloy offers a mininmum tensile strength of 125,000 PSI, while the zinc-plated steel offers a minimum tensile strenfth of 65,000 PSI. What's more important for overall stiffness - material type or diameter?

Also, B7 alloy has a plain finish, while other has a zinc finish. The zinc finish is supposed to prevent rust, but will this also increase the short term efficiency of the lead screw?

Thanks!