Hi,
Both are
excellent.
However, the lenses do have their own characteristic "look" or rendering - the FA 43 Ltd in particular having the distinctive FA Limited lens-family "look", which to my eyes is simply
superb.
Compared to the FA50 1.4, I would describe it as having a unique 3D-effect. It also has an incredible level of clarity, fantastic colours in general (including skin tone), and handles micro-contrast supremely well, bar none.
The FA50 has a more straightforward "look" - which is perfectly fine, mind you. Also very sharp and clear, with simply delightful colours that are purely Pentax. In one sense it can be viewed as a more "accurate" or "faithful" lens than the FA43, in the sense that the FA43 definitely does enhance the picture, such that the result often looks better than in real-life.
Whereas the FA50 can be counted upon to deliver an accurate capture, with excellent IQ to boot.
And if you require stronger background blur, the FA50 might suit you better since it can open up to f1.4. Personally, I seldom need 1.4, and find the f1.9 of the FA 43 Ltd more than adequate - but users differ.
Do note though, that 43mm or 50mm focal length results in one particular style of portrait photography, and you need to first ensure it's what you want. Many portrait photographers also use longer focal lengths, eg. 85mm, or 77mm in the case of the FA 77 1.8 Ltd (the sister of the FA 43), which is truly a portrait lens par excellence!