1050 wire would be the best choice for welding 1050 and 3103 base metals with regards to weld ductility, colour match (polished or anodised finish), fatigue life, corrosion resistance.
4043 is easier to use in that the 4xxx series wires are more resistant/less prone to solidification/hot cracking. 4043 is stronger but is also more brittle, strength typically a mute point as welding Al results in an annealed HAZ i.e. area next to the weld is softer and weaker than unheated base and weld bead.
When welding 1xxx or 3xxx series alloys to a 6xxx series alloy, say a fabrication using 1050 sheet with 6063 tube (header tank, radiator etc) then 4043 is the better choice for the dissimilar welds. 4043 turns a manky grey colour when anodised, if weldment is to be anodised then 5356 for the dissimilar welds
Thanks guys...very interesting to know the detail. I went for the 4043, as I'll be making a plenum, so the strength isn't an issue (I hope...), and I'm tight!