More flexibility in low light performance and range both wide and tele with option #1. On the 600d you would have had to find a lens that is 15-65mm to match the same focal length of 24-105mm on 6D. DOF of f/4 on 6D is also compared to f/2.5 on 600D. The boost in ISO performance and overall look of the images is also better with full frame, but I wouldn't spend that kind of money for street shots if I could barely afford it, you should make the bigger purchases if you know your finances can take the hit or if it will be an investment that pays for itself short term. But of course, in the end, you still buy what you want because it's your money.