You can't really compare Mantle to PhysX.
This is true. I think the point is, though, for something to be considered a value added feature it has to have a lot of actual games supporting it. I've always maintained that I think it's awesome for AMD to attempt something similar to NV in creating value added features. Physx has a lot of titles, while Mantle doesn't. Yet.
I think AMD is attempting to get Mantle into many of the cross platform engines such as UE4, Cry Engine, and so forth. That will take time, and it will take additional time for those efforts to be realized with actual games. Now, don't get me wrong - BF4 is a huge win for AMD in terms of getting Mantle out there. But it's only one game (even though it will be a huge game on PC).
So the abridged version is, Mantle will be considered a value add in terms of purchase decision when a lot of games support it. AMD has work to do there, and hopefully they can get it done - but in the meantime, I don't think anyone will buy the 290X at launch based on Mantle. Mantle is a long term proposition. Not a short one. My take is: I think Mantle is an awesome idea. But I would not make a purchase decision on it until a lot of games used it. That's why i'm personally judging the 290X next week by DX11 only and nothing else.