If I am on a budget, I would get the Pentax DA 50mm f1.8. If I am on an even smaller budget, I would get a Pentax M 50mm f1.7
The FA 50mm f1.4 is probably better for portraits. Hopefully you get it with the nice lens hood. It also has DoF scales and aperture ring, which can be useful. And it probably has better overall construction. So it has a couple advantages over the DA 50mm f1.8. Just set camera to Av, set f1.4, jpeg mode to portrait, and you have insta-art portraits.
The DA 50mm f1.8 has the advantage of rounded aperture blades (might not really be significant, but some people like it) and maybe has better lens coatings (though I doubt there is a difference). I think the DA 50mm f1.8 does not come with a hood.
Where I live the FA 50mm f1.4 is significantly more expensive than the DA 50mm, so I would probably get the DA. Though, I use my M 50mm f1.7 (no AF, no A-mode), and don't think I need another 50mm. But the FA 50mm f1.4 offers you the most. The other portrait lens that Pentax makes is the DA* 55mm f1.4, but that one is a little higher priced. The DA* 55mm is apparently
made for portraits, though. You might also want to look at the Sigma 50mm, but that is a huge lens. You can always check the
lens reviews section of the forum or the
sample photo gallery. The thing with 50mm lenses is that you almost can't go wrong in terms of optics - all produce great images. Its other features that can tip the scales.