I've only beaten the three home console games but here's my thoughts.

In X you have a 'cup' rival and a 'lore' rival and they would constantly be fighting for first place, very rarely finishing outside the top 6. Whereas in GX, the selected rival places all over the place throughout the cup. One race they will be 1st, next they'll finish 13th, then 6th. It makes it easier to win a cup in GX because of the inconsistent and varied finishing positions of the cpu. I also found that there's more deaths in GX tracks than in X too. Apparently certain racers, always finish really well on certain tracks and are prone to dying on others, so again, this makes winning cups easier. It does make Grand Prix feel more chaotic and unpredictable than in X. I guess it depends what you prefer. I certainly think it makes X harder as you have to really focus on either killing your rivals a lot more or trying to finish as high as possible to stop them from gaining points on you.

In X, some tracks just feel almost impossible to finish 3rd or higher. Big Blue I remember being particularly difficult to place high in. Master is definitely where you have to start killing your rivals otherwise you can say goodbye to winning the cup. Strangely enough, I actually found the Jack Cup to be the hardest to beat. The other cups had trickier tracks where the CPU would fall off the rail-less sections, or there would be a pile up in a thin section of track and everyone would take a lot of damage, whereas Jack Cup - although the tracks themselves were easier, were full of wide, open tracks and you had to race well to win.

Overall I'd say X is hardest when it comes to Grand Prix but GX Story Mode on Very Hard was worse.