Thank you for the correction. I have used the 700 before with the 900 and the size difference was noticeable so when I compare the 900 with the 568, the 900 is taller but the 568 feels heavier in the hand. That might be an illusion because the thickness of case on the Yongnuo feels so much denser and thicker than the Nikon SB800. I never weighed any of them. I just got two out and extended the 900 is about 1/2 in taller, and when folded for on-axis flash, the difference is a little more than 1/2 in. Weighting both in hand, they seem about the same but the feel of the 568 gives the impression of being harder, more rigid, denser. I don't have a SB700 here to compare but from memory, I always picture the 700 to be smaller, particularly in the head cross section.
One thing the Yongnuo has over the Nikon models is the more powerful and larger focus assist light. It seems brighter and sharper than the Nikon, plus covering about twice the area. The near IR output of both brands is very effective yet does not distract subjects or trigger blink reactions. They do not seem to see it, or notice it where as the body mounted white light is a mood killer and it is good that it is being dropped on higher models.
Two other differences, the flip out diffuser screen on the Yongnuo is lighter weight weaker plastic, two of mine broke within a month and I do not even use them.
The second difference is about 1/3 stop more light emitted in the same scene as the SB900 so when combining them, say in tandem mount in a folding stripbox or softbox I set one -1/3. All three are identical so it is programming and I have not bothered to figure out which is right. Makes no difference, except with shooting in parallel. 2 combined in a 80x80 cm or large softbox makes a very competitive light source to a 300-400w/s monolight.That means I can do real studio type sessions with a backpack and folding softboxes, with light stands strapped to the sides of my backpack. I have seen adaptor plates that allow 4 speedlights to be groups in a 2x2 grid. So many cool adapters, controllers and accessories out now where someone can do serious work for very unserious equipment prices. I have not turned on my home designed and built strobes for over a year. I built them when I came across some cheap surplus aircraft strobe bulbs so built two 600 w/s strobes.
Looking at them closely I seen another difference that might account for the 1/3rd stop difference. The SB900 has clear plastic lens in the center and fresnel lenses on the left and right of the center. The 568ex has similar fresnel lenses on the sides but also through the center. That would make a different in focused light level in the very center, and also throw.. The 568 focuses a beam width out to 105mm equiv. and the SB900 can focus to a narrower 200mm field of view, but my style seldom uses direct 200 mm focused beams. For wildlife photographers that could make a difference, but my mostly people shots are diffused and modified, even if using 200 mm lens focal length. If I need to light something further out, I want it lit, so put a speed light near the subject controlled remotely so I have control of the light. Placing the light near a distant subject means I have the same options to focus the light or snoot, or flag it as up close, instead of lighting everything between the camera and subject, with the least being illuminated being the subject.