Thanks for the suggestion, but I am focussing on zoom lenses as I will be using it in the same manner as the Sigma 17-50 - main travel lens. Now the Sigma definitely did not reduce the background to a total blur (even at F2., I am just trying to find out if the F4 with the longer focal length (70mm+) and full frame combination will at least match that???
The 24-105 f/4 on full frame is dof/fov-equivalent to 15mm-65mm f/2.5 on APS-C. So it should be quite a bit better on full frame (for shallow dof effect) than the Sigma is on APS-C.
The 24mm-70mm is a faster lens, but is also a little short for portraits. I don't own the 24-105mm but know its reputation and I think it would be a really solid choice as a travel lens. For portraits, it's a personal choice but on full frame if I had to use one of those lenses, I'd take 105mm f/4 over 70mm f/2.8.