The Lenses
The two lenses share a lot in common, but differ in several key ways. Both lenses are constructed with a solid metal shell and are roughly the same overall size. The new XF 35mm f/2 tapers towards the end of the lens and is very slightly shorter, so it looks smaller than it really is. The 35mm f/2 is a bit more solidly constructed than its f/1.4 brother, and has some weather resistance as well, but both are well-built.
The biggest physical difference between the two is the focusing system used. The f/2 lens is an internally focusing lens, so the lens always stays a constant length. The f/1.4 lens uses an extension focusing system, so the front element will move in and out as the lens focuses.
Mechanically, the focus motor in the 35mm f/1.4 is probably the loudest of all X-series lenses. The new f/2 lens has a completely silent and very fast focusing motor. The f/2 is definitely a faster performer in the autofocus department, and in my experience, hunts a bit less in very dim light as well.