I would personally suggest aluminum (maybe thats because I work at an aluminum fab plant). It is considerably lighter, and you won't be seeing any temps that could make it bend.
Then this is all assuming that you weld aluminum, I would suggest taking your time with a nice tig for something like this.
Because aluminum is lighter you could can get pretty thick side walls.
I made a wakeboard tower out of 2" OD 1/8" wall tube and it towed two boarders and some people on it at the same time. I know this doesn't apply too much to your situation, but I am just saying that it will definitely be strong enough. For your situation you could go with something like .75" or 1" outer diameter. Here is a good site:http://www.onlinemetals.com/merchant.cfm?step=2&id=71
I would suggest drawing or modeling the piece you are going to make before purchasing any material. If you have dimensions I can see if I can make a piece and hopefully even stress test it (this is what I do as my job in the summers).