Those engineering decisions depend on the service demands and the possible need for anodizing.

5356 will provide the better color match and slightly higher tensiles, easier GMAW feeding

4043 slightly greater ductility, slightly better wetting and puddle control

Read these

http://www.thefabricator.com/Consumables/Consumables_Article.cfm?ID=1286

http://www.alcotec.com/atasap.pdf