17-55 vs 15-85 vs 24-105L

Just buy the lens for your needs now. Worry about a lens for FF when you have one. Buy used and you won't lose anything but pennies when you decide to sell. There's no one perfect lens unless you shoot one subject exclusively. There's trade offs that need to be decided.
You don't mention landscape anywhere so you don't need anything wide. You do shoot people so maybe a longer focal length would be better for you. As I mentioned in your FF thread how about a 70-200 F4. Killer sharp for street stuff then stick with the 50 F1.8 for indoors in low light.

There are a number of ways to go and I have went with a bunch of them...

First off get a flash and the ability to use it off camera. I'm not sure if the camera you have has a built in master like the 7D, 60D, T2i and later Rebels, but the new Yongnuo 622's sound good from what I am reading and they are pretty cheap.

1. Get the 24-105/4 and use the 18-55 for the wide stuff. The 24-105 replaced my Tamron 28-75 and I used the 18-55 for the wide stuff when needed then I added the Sigma 17-50 later. You are probably going to want to add a prime or two down the road with this option too.

2. Get a 15-85. If you go this route, you have enough left over compaired to the 17-55 or 24-105 to add a fast prime like the Sigma 30 or a 50/1.4 to help make up for the slower variable aperture of the 15-85

3. Get a Sigma 17-50 and a Canon 85/1.8 for the longer stuff. As good as the Sigma is, I would have a real hard time spending ~$500 more for the Canon 17-55/2.8.

My personal vote, because I like having more aperture options available to me is #3.