"Yes, the Sigma is pretty good. Myself and several other forum members are satisfied owners. Take a look at the review on thedigitalpicture where it spanks the Zeiss and the Canon f/1.8 -- it's second in image quality (and a close second) only to the 85mm f/1.2. AF speed on the sigma is quite reasonable."
I have all L glass and Zeiss...But I chose the Sigma 50mm f/1.4 as well..it performs perfectly for my needs...I am always pleased with the sharpness and the bokeh. I think you as a night-time street shooter would enjoy the Sigma..the focus speed is very good as is the bokeh..it is also mechanically refreshing to the two lesser Canon offerings, (did I really just say that about a Sigma product?!
).also..the cost..right now it can be had for $450 with rebate. Sigma is upping the ante ..their new 35mm 1.4 outperforms the big guns for much less money...I like to see more of this!
I skipped the 50mm L..to go for REAL shallow DOF..
I would not regard my 85mm f/1.2 as a street-shooting lens, though...but oh if you have the money and the patience for the slow focus and infinitesimal, shimmering DOF ...it can leave you speechless! This lens is pushing the limits of physics..and has tons of fringing...but LR cleans it up nicely.. Maybe try renting one and see for yourself...but try to move past the obvious difficulties of using this beast...it has great rewards for the patient explorers!!!
The beast also delivers incredible results @f1.4-2.0 and beyond, stunning actually.....it's slow on the focus though...in some cases, maybe better off with the Sigma 85mm f/1.4 or perhaps a Canon 135mm F/2L...alas..it's all a compromise!