If you do a Google search for "7D vs D300S" you'll find this has been discussed many, many, many times before. My first question is do you already own lenses for either system? I ask because neither is a typical first DSLR purchase. The typical buyer will be someone upgrading from a lower grade Canon or Nikon. If you already own a bunch of Canon glass, the 7D is the natural choice. If you have a bunch of Nikon gear, go for the D300s unless you are willing to sell all your stuff to switch systems. If this is your first DSLR, neither is especially first timer friendly as the both lack the typical pictogram modes found on entry-level DSLRs. That said, the Canon 7D at least retains a beginner friendly "full-auto" mode which makes it more "beginner friendly" where as the Nikon is pretty much a pro-grade camera with a 1.5x crop, rather than full-frame, sensor and lacks an "easy button."
If you are an experienced SLR user, the Nikon has a lot going for it. It's AF system is still offers advantages over the new Canon in that it covers more of the frame, has more points and functions faster/more reliably in low-light. I personally like the fact the Nikon viewfinder gives me indicators for the metering and exposure modes in the viewfinder where as I have to look at a dial or LCD panel on the Canon. The Nikon also has dedicated buttons/dials for these settings as well as the AF mode where as they are accessed via modal buttons on the Canon. There is simply more information in the Nikon viewfinder and it's possible to change nearly all critical settings without taking your eye out of the viewfinder. This really is a professional's camera and it's a pleasure to shoot with one. Sounds like I own one...Doesn't it? Well, surprise, I own the 7D.
The Canon isn't necessary "better" than the Nikon but for those of us who shoot XXD bodies and are heavily invested in Canon glass, the 7D finally addresses all the areas that left us jealous of Nikon D200 and D300/D300s shooters. The biggest issues were that we wanted an updated and more versatile autofocus system, better/more accurate metering, and better build quality with more weather seals. The 7D, as mentioned earlier, retains a "full-auto" mode that makes it "easy" for new DSLR shooters but the custom setting positions on the mode dial are a handy feature that let's you customize the camera to your own personal liking. Speaking of customization, the 7D allows users far more options for assigning button functions than previous Canon cameras. The bottom line for me is I would have sold all my Canon gear and would be shooting a Nikon D300s today had the 7D not come along when it did. All Canon has to do now to keep me from defecting to Nikon is apply all the 7D's technology and feel to a full-frame body so I can sell my old 40D.