Your question is above my paygrade but I'll add this much; It really depends on service conditions and design engineering.

The 6063 does have a substantially lower tensile strength than 6061.

This lower tensile strength might be problematic considering post weld strength reductions to be found in the HAZ.

Welding Aluminum: Theory and Practice   by the Aluminum Association has good data.

Ultimate Tensile Strength (psi x 103)

6061-T6       45
6063-T6       35