I have both. For my eyes the Rift's picture quality is slightly superior in a number of comparisons to the Vive's, primarily; larger visual sweet spot, less noticeable screen door effect*, and less pronounced godray artifacts. Both headsets have slightly different "issues" with displaying all blacks correctly (I personally have no real issues with these artifacts). I would also add that the Oculus' superior ergonomics and lighter relative weight generally means the picture stays stable and in focus. You set it once when you put it on and rarely have to readjust it until you need to take it off. It's so. Damn. Polished.
With the Vive you are more often going back and making adjustments, and the headset is more prone to slight movements and nudges, especially when you whip around. You can achieve similar levels of stability compared to the Rift however it requires your headset to be strapped tight (the exact opposite feeling of wearing the Rift). Even then however in certain situations, such as looking straight down, the view may still tilt away - this is just the ergonomics of the head strap solution (which can be detached or jerry rigged with welders mask frames and the such). Some of it is simply the headset being slightly bulkier and heavier. And obviously the nature of roomscale VR contributes as you are much more likely to be looking and moving around.
Whew! That was a lot. Now, that all said - and like with the DK2 before it - it doesn't really matter. Oculus is the slightly superior viewing experience, and the Vive has it's little niggles, but Vive also brings roomscale. The niggles aren't that big of a deal, and roomscale is a big deal**. It's VR! You put on the headset and soon it all melts away as you become immersed in experience, man.
* Personally I find the current SDE hierarchy goes: CV1 > GearVR > Vive > DK2
** I can't wait to see what Oculus has in store with Touch!
