Test SnPb vs SAC and you'll see a difference.  My colleague back in the days when we were still in denial about the transition to Pb-free did a similar test attaching the wires to his wall and using paper clamps on the end (not much weight).  They didn't break, but kept elongating, to different lengths.  Now, if a heavier weight was used, which would break first?  How much weight to break each?