I just spent last weekend running my ck63's as center x/y and ca-14 omni's with 3ft split at Homegrown on the River music festival. While I am just getting into the processing, I think I can hear the difference from when I ran the same setup with the ck61's at last festival. It appears that the 63's may provide a tighter sound that works well in conjunction with the split omni's. I am hopeful to find a clear difference and better understand which one suites which occasion.
Good to hear your thoughts on this, and your conclusions based on that comparison. My take is that when combined with the omnis, the more directional center pair often works best. Sort of a shared workload with each mic pair contributing what it does best, covering for the deficits of the other pair in turn. Increased pattern diversity providing strength in unison.
At the same time, when used as a single pair, and in the appropriate situation, cardioids may sound better as they are something of a middle-ground compromise between the two extremes. As cardioids, they have somewhat more omni component in them.