There are many choices for a filler metal for this type of joint. Qualification by testing would be needed on any selection. A514 can be welded to plain carbon steels with filler metal compatable with the A514 or the A53 provided you cointrol hydrogen content.

That would allow you to test your procedure with either. In many cases the higher strength filler metal is chosen but I do not know why other than some "welding wives tale" or the thinking I state below. I have welded this material to plan carbon steel using E71T1, E71T11, ER70S-6, and E7018 .

If I was welding on the floor and the bulk of the work was A514 to A514 with only an occasional weld to A53, I would want the welding people to specify the higher strength weld metal just to prevent changing wire but that would have to be weighed out against the labor and material costs.

Have a nice day

Gerald
http://www.weldinginspectionsvcs.com