Well...
Let me say I am biased toward Nikon equipment an have been for the past 30 years, and the D300S is just superb -- it has much more upgraded than just the addition of video. I bought one as a backup to my D700 and the IQ is virtually identical up to ISO 1600 or so...
THere is more to a digital camera than just the hardware. Nikon has always had, and continues to have, the much better post processing software, That is an important consideration for anyone that doesn't already have something like Adobe Photoshop.
But the Canon 7D is also a fantastic camera, and if you like Canon gear and want to slant your future collection that way, then the 7D is a top of the line camera.
IF you are looking at video as an important consideration, the 7D has better sound -- the Nikon D300S has very poor sound built-in, and you will need an extension mic (they attach in the hotshoe). The Canon is better out of the box for sound.
I would suggest you find a camera dealer and go play with both of them for awhile. The features, in reality, are about identical. Usually, it boils down to which camera system/buttons you like better. Canon users tend to prefer the canon layout on the cameras -- Nikon users will usually prefer the Nikon layout (especially people moving from SLR's to digital SLR's - since the button layout is virtually identical)...
GOod luck with your choice!