I believe it depends on how the season progresses. Mercedes will always be the quickest in that first season, but whether they have more mechanical faults, Ferrari stays on par or not, etc. just depends on how the dice roll.
In my career mode, Hamilton has not retired once, but had one pretty bad race. Rosberg has retired multiple times but has won more. While Ferrari are nearly on par with Mercedes to the point where at most races they (usually Vettel) can challenge for the victory and can come out on top with a bit of luck or advantage for that circuit.
