SRR is my guess. Charles was a 20-year old boy who'd had around 25 fights. SRR was peak, having lost 1 out 125 fights. At LHW a peak Charles would be way too much for any version of the smaller SRR.
SRR as he faced better competition for the time,but if Ezzard actually managed to rack up some more experiences with elites or even the murderers row,he could have a better chance. But I still don't see this fight as a one sided steamrolling,young Ezzard is already talented and skilled.