At today's prices, by one of each, and find out for yourself what you tend to grab the most often.
Then you have your answer. Sell the rest, and maybe get a second body.
For TTL fill flash, the FA gets you as close to perfect / easy as I could imagine. I have a pair of em, and an SB16. Super easy - but no better than a good thryristor external eye system, unless your doing macro.
I shot a lot of fill flash using manual calculations before the days of TTL, and once you get the math down in your head, it is really straightforward and easy.
===== manual method ======
Just set the camera to speed it syncs at. First, turn the ASA speed up 2/3 - 1/2 stop OR set compensation for 1/2 stop under exposure ( critical for reversal films, so so with Negs )
Adjust meter on camera for correct exposure ( say, F8 for example ) then set the flash to give you a 5.6. Poof... Perfect 3 - 1 ratio.
( I know some people will say that is 2 to 1. But they would be wrong unless the flash is ONLY illuminating what the ambient light is NOT illuminating. )
Here is the explanation. ( using this method makes it easy peasy )
Lets say that the f8 exposure is for X units of light. Setting the flash for 5.6, means that it would expect the LENS to be set to f 5.6, so the flash exposure would be 1/2 of what you need, or 1 stop under-exposed at f8.
So f8 = 100% Units of light required.
f5.6 = 50% Units of light required.
Shaded portion gets only 50 units.
Portion lit by flash AND ambient get 100 + 50, or 150 units of light. 150 / 50 = 3. 3 to 1 ratio.
See that 150 Units for flash and ambient? That's 50% more than whats needed, so that is why you set exposure compensation at beginning so you don't overexpose if your shooting chromes.
1 stop difference between flash setting and ambient is 3 to 1.
2 stop difference is 5 to 1. ( 125 units / 25 units = 5 to 1 ) And you only need to set exposure compensation by -1/3 stop for chromes.
Now when you are using your rollieflex or classic blad or whatever, you can do the same thing with ease, and get the big lovely Chromes or Negs, perfectly exposed and with a 3 to 1 ratio on the critical parts, with the background going slightly darker but only a bit, and retaining detail in the highlights.
=== edit ===
I read the above comment that the FA does not really do fill flash, and that is true - it does not do adjustable ratio fill flash automagically with some button or knob. if your shooting at a 250th, and set your aperture accordingly before you switch the flash on, the ttl metering would give you about a 2/1 ratio, and the ambient + flash is going to be more than the flash illuminated only. However, to get other ratios, If you manually meter, and configure the camera to the correct ambient light, and then dial in say, -1 exposure on the compensation, the camera will compensate, and underexpose the flash by that amount. So it' kind of a backwards approach from what I described above for pure manual calculations. I guess I been doing it the 'hard way' so long I forgot that its not all as "automatic" as some would like...
Blaine