The comparisons between 08 an 09 models will pretty much leave you looking at size, clearance, and features. With exception to the Outback XT (turbo), the 2.5 L drive train is the same, and both cars are on the same platform. Both are great in snow, but remember that AWD isn't a silver bullet, so I'd recommend snow tires for either car in the winter.
Features: The OBS is a version of the Impreza whereas, while the Outback is still a Legacy Wagon, Subaru treats it as it's own model line, so you will have more vesatility in what you can find for options on an Outback. There is the 2.5i, the XT, limited editions, etc. that all come with cvarying option packages. The Outback sport is a version of the Impreza that comes as a set option package. It's still very well-equiped, but you won't find an OBS with leather or nav, for example, unless it's aftermarket.
An important note - the Outback Sport, compared to the Impreza comes down to wheels and 2-tone paint, the advertised "Increased ground clearance" on the OBS is only .2 inches and comes from having 17 inch wheels vs. the 16 inch on the Impreza 2.5i. If you don't care about 2-tone paint or wheels, you can get an Impreza 2.5i hatchback with the same options, probably for less.
The Outback, when compared to a Legacy includes the wagon body, different suspension, and probably more stuff that I'm not aware of.
When you get to the '10 Ouback vs. the '10 Outback Sport, worlds apart. The Outback is on a new platform, has new options, new drivetrain components...someone else would have to tell you more about this.
I have an '09 Outback Sport and love it. I like the paint, the wheels, and having something a little different than the usual Impreza 2.5i. The difference in ground clearance doesn't really matter to me as I drive predominantly on tarmac, and I don't need the extra space. Or at least I didn't when I bought it, now that my wife is pregnant....there may be an 05-09 Outback XT or Legacy GT in my future...or in hers.