It is very open-ended. Horus wouldn't necessarily be the Emperor's favorite because they weren't alone for so long. Others wouldn't be resentful of their harsh lives and might lack the skills those hard lives taught them. It all comes back to the nature vs. nurture argument. I believe nurture plays a bigger role in determining who a person is over their inherited genetics. The nature does play a role but it is secondary.
"Fetch me another plaything. This one seems to have broken." -Urien Rakarth
Space Marine- "This planet is ours, witch."
Eldar- "No ... This planet is theirs ..."