Unfortunately, I cannot speak from motion sickness experience, as I've never experienced motion sickness, but from comments I've seen on FJ in the Universal forum, a lot of individuals do make comments of being ill after going on the ride. However, some people on this thread state that themselves been on FOP and experienced sickness, but not on FJ, which I find interesting.
That being said, just simply from experiencing both rides themselves, both rides are smooth and graceful in terms of movement,
however, FJ is more drastic and dramatic in terms of movement due to the flexibility in the robotic arm and doing more dramatic maneuvers (at one point you almost go upside down, but do not), where as FOP you're attached to something that more is in line with a stationary bike and your movement is limited in comparison to FJ.
Also, for example, I noticed
@Laura527 stated she was ill on FOP, but not FJ, the reason might be due to the 3D elements, but non-3D elements of FJ. Which implies it's the screen and actions of it is a contributing factor as well, not necessarily motion. I back this claim up with being a contributing factor due to the fact that other FJs that were open in other parks ended up adding 3D as an additional element to the ride (as they were created after the original FJ), only to eliminate the 3D feature due to substantial increase reports of motion sickness in those parks.
It really depends on the person unfortunately and the combined elements of the ride.