I already have Sigma 17-70mm f2.8- 4 lens and thinking of adding a pancake lens to my lens bag. Is it a good choice for overall full-body portraits and general street photography?
You can answer this question for yourself better than anyone else can. Tape the zoom ring of your Sigma 17-70mm f/2.8-4 at 24mm and go out and shoot some full body portraits and general street photography. The only major differences will be a slightly narrower maximum aperture (Zoomed to 24mm the Sigma is already at around f/3.2), and the larger size of the lens. That should be close enough to judge if the focal length is appropriate for your intended usage.
If you want to compare the optical performance of each lens, have a look at The-Digital-Picture or DxO Mark. The Canon does a little better than the Sigma at The-Digital-Picture. Tested on the 70D at DxO Mark the Canon pancake does a bit better at f/2.8 than the Sigma at 24mm f/3.2, but not by any significant amount (see below). This holds true pretty much across all apertures tested. Tested on the 700D/Rebel T5i the Sigma does a smidgen better than it tested on the 70D while Canon does about the same as tested on the 70D. So on the 700D they're pretty much dead even.
Copy-to-copy variations are probably just as wide as the difference between the particular lenses of both models as tested by DxO.
Since both of these lenses are for APS-C cameras, you'll get a field of view similar to a 40mm lens on a full frame or 35mm film camera. 35-40mm has long been a favorite focal length of many street photographers and for use by many for full body or environmental portraits.
About the only thing you'll gain by using the EF-S 28mm f/2.8 pancake over the 17-70mm f/2.8-4 Sigma is the smaller, lighter lens that will be less conspicuous as well as easier to carry around. Of course you'll be giving up the ability to shoot as wide as 17mm and as long as 70mm in addition to being able to shoot at 24mm focal length.