I've only played each course once...but they were both on the same day. It was an admittedly brutal 36.
I may be in the minority, but I don't have strong feelings about the Castle. It was a fun play, but it wouldn't be what I'd go to Scotland for. We played it because there was a gap in the schedule and it was convenient to where we needed to be (Kingsbarns after more of our group flew in). The course just feels like A LOT, but not in a natural way...making it feel very American. I didn't think the greens were too wild in general, but I found a few cases where the severe greens and the relatively narrow fairways clashed. The place I really would have liked to attack the pin from was often 20-30 yards into the weeds.
I liked Kingsbarns, but I think the shortcomings have all been spelled out here pretty thoroughly. I do like that both 9's traverse the land in both directions from the clubhouse, unlike Castle. The returning 9's feels pretty American in both cases, but it feels a little more like happenstance than pure planning at Kingsbarns. Kingsbarns wins on feel, but if I went "hole by hole", it would win over Castle pretty handily as well.
A non-GCA feather for Kingsbarns: they stay open for dinner for the last group coming in, which we were. This was my first time full day in Scotland, and we weren't aware how difficult it is to find substantive food after a late round.
The cost is too much, though. I'd rather play either Elie or Carnoustie than Kingsbarns, but for the price you can come close to playing both of them instead. If we're just comparing to more modern courses, I strongly prefer Castle Stuart to both Kingsbarns and Castle.