Hi Tonkotsu,
Let me tell you what I think about it. I have an A77 with the old Sigma 30mm F1.4 and theSony 16-50 f2.8 lens. I got the sigma just to make my image less noiser under night time urban street photography(oddly, it ended up working a lot more for nighttime video later as opposed to my initial purpose
It worked. As A77 is not so great for low light high iso performance, I desperately need fast glass to control the iso at 800 and below with F1.4 with the main lightsource being streetlight a few meters away from the subject(with a cooresponding shutterspeed of 1/50 second, which seems to be the bottom line to minimize the motion blur for slowly walking subject). With the 1650 sony, the iso would have to be as high as 3200, which I would avoid at all cost since the drop in image quality is obvious.
My second intention is that I want to get shallower depth of field compared to F2.8. However, to get decent amount of background blur, I find it crucial to keep myself as close to the subject as possible(1.5-3m is the working distance for sufficient bokeh in my experience). Now you already got an F1.8, I guess it is not too far behind in terms of Bokeh(since the focal distance matters a lot more for wide angle lenses if maximum bokeh is what you are after)
However, a downside of my copy of sigma is that while the center is sharp enough for my shooting even at f1.4, I find it extremely difficult to achieve critical accuracy of focus when I wish to place the subject somewhere other than the center of the frame due to its mediocre sharpness of the border@ F1.4. The critical focal plane for anywhere other than the center will be as thin as your fingernail so unless you make precise focus the image would look soft if you wanna frame your subject at the border or even mid section of the picture. Also, while it focuses well with center point with my A77, I find other focusing points failed to deliver the precision for the ideal image that I mentioned above. If you are going after that sigma, you may face that problem too.
Another issue is that for the above purpose I need to move quite close to my subject when I shoot with the 30mm sigma. Most of the time I find it more difficult to get a nature-looking shot with a 50mm-equiv lens as they almost instantly notice that I tried to take the picture of them and the mood and situation that I wish to capture are usually gone. I later got Samyang 85mm f1.4 for that purpose and find the subjects looks more natural as I took the picture at a further distance. But again, if you primarily took picture of your close friends, relatives, or, you are a better communicator than me which grant you more pass to candid photography for strangers, you will love the intimacy that a normal lens will offer you, which you already could achieve with your 35mm f1.8 sony lens, a lens with much higher resolution at the border of the image than my sigma. Also, for close-up of still-life like food and plants, the 35mm f1.8 will fare much better than my sigma as it has 1:4 reproduction ratio while my sigma simply cannot do it with a nearly useless 1:10 ratio of maginification. Also, another difference is the price, you have to pay some more for the sigma to get the extra 2/3 stop advantage.
I handled a copy of 35mm f1.8 for a few days before I got the sigma and I'd say the build quality of sigma is better as the focusing ring is properly rubberized and moves smoothly when doing MF. However, for A mount, the Sigma is no more a silent lens as there is no HSM for Sigma 30mm sony version and it is actually quite noisy when AF(it is quick, anyway) and even makes some noise when MF. The situation of sony lens is not much better, though.
So it really depends on your style and purpose of shooting as the sigma allows you a litter wider and brighter but you may find its softness of the border part of the image ruins the show sometime. Also, you have to pay some more while giving up some of the strength of the sony 35mm. Also, you may also think if the sigma worth the extra money as you will have to give up the robust close-up capability of the sony 35mm(I suppose you will sell it to get some money back after the deal). And the bokeh thing for both lens really lies in closer to the subject while not that much with the difference of aperture(they will look both a lot better than DT1650 at the same focal lens and it does make a difference, though)
Now, your time to make the call ! Sorry it's a bit long but I hope it helps!
McLuhan
