I have both the DA35 and FA31 and think they both make great lenses that will give you a FOV close to an old 50mm on film. The DA35 is more compact, has a nicer hood, does closeup very well and has quick-shift focusing which is really nice. The FA31 is faster, and it's the FA31
All kidding aside, you can't go too wrong with either one.
But as deadwoldbones pointed out, neither one will "replace" your 16-50. However, if you don't think you're likely to need that zoom range much, you have wide nicely covered by the 12-24 and long by the 50-135, so a nice 31/35 to bridge the two lenses might work well for you.
One other thought. Since you have the 12-24 covering the wide, you could also consider the 17-50's older brother, the 28-75/2.8. I find it to be a great range for people picture taking and the IQ is excellent. It's one of my favorite lenses.