neither is heat treatable and so you could not restore full T6 condition if you wanted to. The 5356 has a much higher as-welded strength than 4043.   If your part is to be anodized, you may prefer the 5356 because the 4043 will react to a dark grey or black which may be cosmetically unappealing for your application.

If you need to restore T6 condition then use 4643, but you will have similar anodizing reaction.

Neither 4043 or 5356 are heat treatable fillers

4643 is designed especially for heat treatable alum

There are other differences...... Tell us about service conditions, color match requirements.. Ductility needs etc.

As mentioned neither 4043 or 5356 is heattreatable. However, depending on joint design, parameters, etc. The weld metal may see some increase in strength due to base metal dilution, if the material is solutionized and aged. Between 4043 and 5356, the 5356 will be stronger in the as welded as well as the aged condtion, due to the high magnesium content. If higher strength is needed then you may need to move up to a heat treatable alloy such as the 4643 alloy. 4643 gives good weldablitity, similar to 4043 due to the high silicon content, and according to Alcotech, can give up to 90% of the basemetal UTS if solutionized and aged. I acutually performed a tensile test once using 6061-T6 and 4643. After solutionizing and aging I was able to pull 41ksi or 91% of UTS.