I have Core XR6 8m and GTS5 10m, and I also have the nexus 7m and tried 9m as well.
XR6: it is the most user-friendly high-jumping kite i have ever ridden. I topped my jumping game from 9+meters into comfortably jumping 12-14 meters in just few sessions. The reason ? It pulls longer/higher and lands softer with more hang time (more confidence). Its hard to over-send because its not so quick anyway.. Also wind-range is huuuge. When you go really overpowered, you get like 15-20 more degrees upwind than GTS/Nexus, because the kite will go further to the edge of wind window. So I could ride my 8m in 30-38 knots without any stress or fatigue. I don't agree on high bar pressure - Its actually pretty light (as on every Core kite I have tried). Drift - is bad, looping is OK, but I probably wouldnt do high jumps with kiteloops on anything bigger than 8m. XR6 is a good old-school trick kite.
GTS: 10m is faster than XR6 8m
It is much more precise and you get much more power in turns. Of course it feels much more cool, but the downside is - you need more skills to get high jumps and soft landings, especially in choppy conditions. I'd suggest to learn it on XR first and then proceed to GTS.
Nexus: its something in between. Not so radical as GTS in loops, but pretty direct. Dont expect higher jumps than GTS, but for learning kiteloops its better, because the pull is not so strong. The biggest plus for Nexus is that its very universal. It has great drift, decent wind range and is good for kiteloop tricks.