Yet in truth the two legions are united in their uncompromising drive towards a singular, absolute purpose. At the highest levels the difference between their respective goals may only be a matter of nomenclature. Perfection may be akin to strength. Imperfection may be very similar to weakness.
In each case the obsession may in fact be driven by a deep rooted insecurity. As detailed in the short story Chirurgeon by Nick Kyme the Emperor's Children were plagued in their early years with a contaminating flaw to their gene seed, which was only eliminated once Fulgrim was found. The Iron Hands labor in the shadow of their Primarch, subconsciously fearing that their true selves may have been too weak to live up to his expectations for them.
Despite their similar goal, it is undeniable that the ways the legions have respectively chosen to manifest their drive differs greatly. The Emperor's Children have delved ever deeper into their experiences as sensate beings, ever seeking to develop and experience more. The Iron Hands on the other hand have seemingly desired to turn their backs on the aspects of their being that make them human, hoping perhaps to deaden the pain and insecurities of their primarch's disappointment by becoming unfeeling machines.