The W is rare because it's the last Fujica screw mount and mechanical camera of the company, it was introduced in late 1978 and after mor eor less one year they introduced their X bayonet.
It has few improvements in comparison to the 705, namely a different tip of the advance lever (that I prefer), the shutter locking mechanism is like the one of the SPF while on previous Fujicas you had to pull the shutter release up and lock it in position, inside the lightmeter looks like the Pentax KX's one and it looks slightly more modern than the 705 but of course the most important upgrade (that I never used, to be honest, because manual winding is pleasant with these cameras) is that it accept the autowinder of the AZ1.
The only place where you can find some info on M42 is this one for the bodies:
http://www.pentax-manuals.com/fujica/fujica.htm
And here there is the list of the lenses:
http://www.pentax-manuals.com/fujica/lenses/m42_lenses.htm
As you can see Fujica never tried to "go pro" with the cameras but in my experience their lenses with EBC coating were top notch, just behind Pentax (with their SMC coating) and way ahead of Canon and Nikon (even if they will never admit that). I noticed that in their fastest lense (the 50mm f1.4 and the 135 mm f2.5) there is some vignetting and the corners aren't as sharp as the centre, but they have less barrel distortion than Nikkors, for instance. As stated the 75-150 mm zoom I have is superb, but VERY heavy.
