In good light, it'll be hard to distinguish a 600D + 17-55 shot from the 6D + 24-105. But if you have to raise the ISO above 400-800 or so, you'll notice the better IQ of FF sensor. Since you mention street, if that's in the evening/night, the 6D will be better. Also, you mention portraits, so be aware the crop factor applies to DoF, too - to get the DoF of an f/2.8 lens on FF, you need an f/1.8 lens on a crop sensor.
Don't forget to budget for accessories. For the types of photos you list, I'd consider as mandatory:
- A good tripod and ballhead. Manfrotto at a minimum, or Induro or Benro
- A good circular polarizer, B+W if possible. Adds contrast and saturation, blues up skies, reduces reflections.
- A 10-stop ND filter. Long daytime exposures to blur people out of architecture shots.
- RAW conversion software. DPP comes with the camera and is free, but not good for library management and kludgy to use. Lightroom is a popular choice.
For the filters, it's a good idea to get them in the largest size you think you'll need, and get step up rings. If you see a 16-35 II or 24-70 II in your future, you can get 82mm filters now (I started with 77mm and step ups from 72mm, 67mm and 58mm, then had to re-buy in the 82mm size later).
My inclination is to recommend the 6D + 24-105. But, you should consider your total budget. The accessories on my list could total $1000. A 600D + 17-55 with a good tripod is likely more useful than a 6D + 24-105 with no tripod or a cheap one.