Since 4501 was a diesel electric made by goodwin alco and a the mikado a steam loco made by Baldwin in the USA, the visual difference would be greatly noticeable. They would have had both the same load ability as per the NSWGR load tables on the southern lines network, since the 45 class was introduced on the southern lines as a direct replacement for any of the remaining 59 class mikado's still in service.
PS if you were asking about a Southern railways of the USA, 4501 there would be no different as IIRC it was a USRA 2-8-2 light mikado so you would just be comparing it to itself thus being a rather silly question and I am sure not what you were asking.