silver:
I'm afraid I can't say that much about the 7x yet since I've only had it about a week or so. It seems comfortable and good for an around-town bag (I actually got it for hiking since I think it'll work fine in front of me while I carry a camelbak on my bag). One negative is they should have made the strap so the padded part is moveable. When you flip it from back to front, the non-padded part of the strap rides on your shoulder more which is not as comfortable. I've taken to wearing it backwards to compensate, but this is something I wished they fixed in the x revision.
The slingshot is a good bag as well...bigger so you can fit more stuff, but you can't carry a backpack with it since it's really a backpack. I have a friend who's happy w/ a slingshot 200 but he carries more stuff around.
So it really depends on what you want to carry and how you want to carry it.
Here's an idea of how snug it is w/ the K10D:
p.s., forgot to mention the Sigma 17-70 doesn't fit w/ the lens hood on. You have to pop the hood and put it on the bottom which is ok because you usually only need the hood in really bright conditions...