I think Ferrari's solution is actually better.
Mercedes' aggressive outwash design appears to be an attempt to provide more consistent downforce at the expense of peak downforce. The flaps direct more air flow around the high pressure area in front of the wheels, but the effective span of the wing is compromised in order to accommodate the sharper bend.
The more subtle bend of the Ferrari design, on the other hand, allows for a longer span, because it relies more on the steering angle of the wheels to direct air flow around them. That means there will tend to be a bigger variance between peak downforce created in turns, where it's needed most and where drag is relatively inconsequential, and downforce created along straights, where it's largely unneeded and induced drag is a penalty. That makes it more efficient.
However, the dependence on that complex interaction between the wing and the wheels also makes it difficult to get right. Ferrari has been
working on it since 2012. But, I suspect there's a good reason why Red Bull, the undisputed Kings of Aero, have copied (and probably perfected) this concept.
As always, of course, I could be completely wrong, though.