yay for welding heat treated aluminium alloy!
would you like me to burn my money too?
steel will be stiffer and cheaper. 6063-T6 that has been welded is 6063-O (annealed)
if its been welded (and of course not artificially aged after) that will be no stiffer than steel per unit of weight.
when will people learn, STEEL IS NOT HEAVIER THAN ALUMINIUM FOR THE SAME STRENGTH or stiffness. unless it is heat treated/Aged.
however that bar might not be welded to the strut tower mounts, in which case 6063-T6 is some goooooooood stuff!