Free Soul, excellent website! You've got a good eye.
I'm in pretty much the same boat as you. I've got a Voigtlander 58mm 1.4, but need a wider fast prime as well. I think the posts so far have had great advice.
The choices I've narrowed my own search to are:
Sigma 30mm 1.4: I've seen some *excellent* portrait/wedding photos from this lens. The cons are extreme border resolution (not really applicable to you), aberrations (can be fixed in post), and lousy minimum focus distance (but good enough for head shots).
Pentax-FA 31mm 1.8: Perfect in practically every way. It's just pricey and there's no way to get around it. You can always think of it as an investment, as its value will likely never go down.
Samyang 35mm 1.4: A brand new lens entering the market this month. Pros are full frame, consistent sharpness across an APS-C sensor (according to the company's MTF charts), and good minimum focus distance. Cons are 77mm filter size and *manual focus* (this is not a con for me, but perhaps is for you).
The Sigma 28mm 1.8 from what I've read is a good lens, but not quite as good wide open as the Sigma 30 1.4, and I've read that it does a better job with close-focus subjects than farther out ones. It does however have the perk of having more uniform sharpness across the sensor. That said, some people love this lens a lot, so your mileage may vary. I'm actually planning on some day getting its cousin, the Sigma 24mm 1.8 macro, but more for artistic shots than portraits.
For the portrait lens, I'll likely go with the FA31 or the Samyang, but I'd be happy with the Sigma 30mm 1.4 as well. Anyway, I hope the stuff I've learned from my research helps a little bit. Take my opinions with a grain of salt, since I don't own any of these excellent choices.
Let us know what you decide and be sure to write a review on it! Good luck!