Both fine wheelsets. Mavic Askiums are also OK and cheap. Alternatively a pair of 32h handbuilts, say Open Pros on Ultegra hubs won't cost much more and will be much more rebuildable.

In terms of slowness, the 32h / cheap wheel option won't be much slower then the Ksyriums since it's aero not mass that matters. Also my experience of Mavic Cosmos versus CR420s is that the ride of the Cosmos is better.

Third option is to fix the AmClassic hub yourself. They are dead easy though to maintain. The only trick is not to overtighten them after putting in new bearings- to do it properly you need 2x 17mm cone spanners so that you can set the rear wheel play with the wheel clamped in the frame. A little bit of play doesn't hurt either. Probably the most satisfying option in the long run.