I have a 605 and a 701. Both are great cameras, compact and lightweight with simple controls and a bright viewfinder.
If you have to choose only one, get the 605 because it takes a standard 1.5v battery and has a split image focusing aid. As noted above the 701 uses the old 1.35 mercury cell.
My 605 and 701 both work with all of my m42 lenses (Helios 44, Pentax 135 f3.5, Vivitar 60-260 F4.5, Helios 40 -m39 with an m42 adaptor).
The 801, which I do not have, supports Open Aperture TTL metering when used with Fujica's proprietary version of the m42 mount lenses.
Here is a chart which compares the various Fujica m42 mount cameras.
http://www.pentax-manuals.com/fujica/fujica.htm