Hey jm,
Personally, I use steel tubing/flat stock for almost all the fixtures & jigs I have designed at my employer. Our largest fixture is a 50" X 120" unit for the welding of the glass sidelight doorframes we manufacture. I use GMAW for all the welding at work and splatter has not been an issue for any of the jigs. There are a couple I had to make from AL because of the restricted area to weld some inside corners & attachment pieces, and (2) fixtures to buttweld 12" sheetmetal shelves for added length. Our company buys C-10 only and .035 S-6 wire and I have not had any significant problems with splatter. Hope this helps your considerations.....  Denny