Interesting design comparison. I overlaid a side view of a 2015 GS350 over a 2015 K900. See photo.
Note: I flipped the Kia image, which is why the gas filler door appears on the wrong side.
If you look back at the 2011 GS 350, there are other interesting similarities. I'll leave that to you to explore.
Given that both cars were introduced about the same time, I wonder if there were any of the design team members working on both cars?
This much similarity can't be coincidence.
My preference is still the K900.