Chromoly is going to weld the same as steel tube. They are both very similar they are both steel its just chromoly has different ingredients so to say and manufactured differently. I would do the tube. if you are not to comfortable solid rod would me more forgiving in the process as you wouldn't worry about melting tube edges. Warping or heat distortion should be less or none with solid rod being its thicker and better able do absorb or dispate heat input as a heat sink sort of.When you get that small as far as weight wall thickness is wall thickness basically .035 steel is same as .035 CM I don't really know if there would be a weight advantage to CM at that small of a tube over regular steel.