The VR ball is the superior VR chair for FBT.

Even without being a coomer, there are good reasons to use it, though you shouldn't overdo it (sitting more than 30m without standing up) to prevent overloading the small muscles stabilizing the spine. But as an FBT user, you should mostly be standing anyways.

Here are some arguments in favour of using a VR ball:

You can sit on them from any angle. You don't have to worry about where it is facing because it's a sphere. There are no obstructing backrest and armrests.
They are safe. If you swing your arm or leg and accidentally hit it, it doesn't matter because the material is soft and actually feels pleasant to the touch. No more hurting your hand by punching your backrest.
You can lay on it as well, giving you an alternate way to rest.
They obstruct the trackers less because they're smaller.
They are cheap. You can get a big one for 20-30€.
You can actually use it for its intended purpose, doing gymnastics, as well.
They are easy to move. Because it's a resilient sphere, you just grab it anywhere and move it a bit, it'll roll away on its own.
They are fun. Bouncing allows you to be more active while still resting, satisfying my hyperactivity.
They save space. When you don't need it, just throw it onto your bed and it's out of your playspace. You can't throw a chair onto your bed.