The siggy 15-30mm won't perform that well in low light but it is a solid glass. It is sharp and IQ is good. It is prone to purple flare, has a loud AF, and common problems that are associated with wide angles and filters/hoods. The Siggy 17-35mm f/2.8-4.0 is not the most ideal glass. It is known to have AF problems and is a bit on the soft side. Users over at FM have rated it a 6.2 out of 10. IMO you should look at these
1. Canon EF 35mm f/2.0. It will basically give you a 50mm focal range which is great for full body portraits but its too long for closeups. It is perfect for low light and works pretty well in landscapes within its limits.
2. Sigma 10-20mm f/4-5.6 EX DC HSM Not that great in low light but this or the canon 10-22mm is an AWESOME lens. Great build quality, vibrant colors, sharp, and fast AF. I have used this lens and it gives a whole new dimension in your shots and its a blast to use.