For portrait work the 50mm will be a better focal length. For general purpose/walk around type lens, the 35mm will be better. On a digital crop body, the 35mm will have a field of view similar to that of the human eye.
For many many years the standard lens that was sold with a film SLR was the 50mm f/1.8. Now in the digital SLR age and the size of the sensor, the 50mm actually has the field of view that is more like a 75mm lens. So based on that the 35mm lens on a dSLR will be the better choice for general purpose/candid photography.
For portrait work a slightly longer focal length was considered by the "pro's" to be better, therefor in the digital age, the 50mm has become a very good choice for portraits.
I don't have the 35mm (I have the Sigma 30mm f/1.4), but do have the 50mm f/1.8 (the older AF version). I love the image quality the 50mm. From the images I've seen the 35mm is also excellent.
So in the end the choice is going to be what you really want it for the most. You can do portraits with the 35mm and it will be better for candids.