The HTC Vive should be out earlier and by all accounts is way ahead of the rift in every way, including no motion sickness from what they have been saying which really is a significant point.
Motion sickness is down to the design of the game/experience, as well as any fluctuation in framerate. Simply, if your head appears to move in VR space without your head moving, the vom starts coming.
If you've got a point of reference like a cockpit round you, and the world is just moving outside that....it's fine, your brain is happy with what it's seeing.
Framerate dips and stutter are a nightmare. Honestly I think one of the biggest differentiators is going to be the launcher software that can get your GPU and OS set up right to deliver a smooth experience.