The S 35mm is a better performer than the 30-90mm zoom. I do not have the 30mm, but I would imagine it is the same story.
The zoom is convenient and very good compared to most lenses you would find from other manufacturers, but the laws of optics have not changed...zooms are compromises, primes are much less so. The zoom is at its best though at that range, and I can see it being a very viable alternative to have one zoom for 30-40mm use if you mostly need it in that range. Then for times when you don't need the extreme corner performance, you also have the rest of the range available. The downsides are that it is heavier than the other two lenses on the camera (though one lens is lighter than two), it is slower, and it does not have the lens shutter option. The ergonomics are great though, and if you are primarily looking at the wide end, the corner and edge performance is not a problem. It will be higher in the primes, but not egregiously so. The bigger problems in corner and edge performance are not visible until 50 to 60mm and above. Even then, in a lot of compositions they will not be relevant.