Also, the actual HMD (excluding strap, headphones, cable hooks, etc.) of the Vive is still about 40% heavier than Rift's, so there's still more "face tug" and fatigue over longer sessions.
Again, I have not directly tested this, but it makes sense as there's substantially more plastic in the Rift's strap system (i.e., back of the head) than Vive's.
Edit: It seems the initial Vive release was heavier than the Rift but the latest release is lighter.
The new Vives weigh about 468g. According to Amazon, a replacement back strap weighs 1.8 ounces (51g)[2]. The Vive HMD w/out strap would be - at the least - about 417g.
Even at a 417g weight, that's about 30% heavier than Rift's strapless weight. Since the 1.8g may include packaging, the HMD may weigh a bit more, bringing the % up to 40% or so.
[1] https://www.reddit.com/r/oculus/comments/47rb3m/the_rift_cv1...
[2] http://a.co/6PUVw7v: "Package Dimensions 4.2 x 3.9 x 3.9 inches; 1.8 ounces" (so 1.8 ounces may include packaging).