I think that you're looking at a lot of extra weight there for the little extra strength that you are getting for your application.  I would think that a pickup sprayer would be an application where you would want to reduce that weight as much as possible.  I know that I looked at sprayer booms at the last couple of farm shows that I have been to, and Bestway's boom appeared to be made out of 4x4 square tubing, and the end looked like it was 1/4" or so.  Looking a little closer, the whole outer section of the boom was a carbon fiber material, instead of steel, which made it MUCH lighter.  They had a piece of the carbon fiber stuff on the table with the same length of steel tubing, and I bet the carbon fiber weighed 1/3 or less of what the steel tubing weighed.  Maybe the boom that you looked at was the same?  Anyway, I could see 1/4" for the inner sections, but the weight of an outer section made from 1/4" wall would require some really heavy supports and cylinders.  I think that you'd be better off to look at some of the truss designs and copy one of them.  There are some pretty simple booms out there that would be lighter and still pretty easy to make with a chop saw and welder.

As far as the round vs. square or rectangular, I think the major reason there aren't many round ones is the lack of flat surfaces.  Everything you mount to a round tube as far as bracing and such needs to be coped, which is a pain in the rear, unless you have a tubing notcher (and I don't mean a fixture with a hole saw, if you're doing very many). 

Anyway, that's my two cents, which you didn't even pay for, so you got a deal.

Good luck and let us know how it turns out, whichever route you go,

Mike