I have been using both lenses for quite some time (FA 77 on K5IIs and K-1 and DFA 24-70 on the K-1). I think FA 77 is an outstanding portrait lens, so if portrait photography is your top priority I guess you should go for it. There is some color fringing in some cases but so far it was easy to correct in post-processing. The DFA 24-70 is much bigger and heavier. While certainly not on par with FA 77 regarding portraiture, it gives you a lot more flexibility being a zoom. I took decent portrait shots with it but I certainly prefer the FA 77.
Let's put it like this. When I go photographing stuff like low light events, bars, portrait session with friends I almost always take the FA 77 and not the DFA zoom, for several reasons. It has a significant low light advantage, it's small and easy to carry but most important I have a specific type of photography in mind and I don't expect to change lenses so often.
When I go hiking in the mountains for 2-3 days, I always take the DFA 24-70 and almost never the FA 77. This is the reason for buying it. It's a lot more flexible, I can do landscapes and occasionally snap a portrait if needed. Most important I don't have to change lenses all the time. Sometimes I get bad weather and the 24-70 is WR.
So it all depends on what you plan to do. If you plan to shoot mostly portraits in a controlled, predictable environment, you should go for the FA 77. If you want more flexibility, weather resistance and you expect to shoot in unpredictable conditions, you can go for the DFA zoom.
Below there are some portraits with both lenses.
FA77 on crop sensor (K5IIs):
FA77 on full-frame (K-1):
DFA 24-70 on full-frame (K-1):