1) The Nook is not officially available to Canada and can't be shipped outside the US (at least, not without involving one of those reshipping services), but if you make a cross-border shopping trip you can get one.
However, the B&N e-bookstore is also not available to Canadians and you'll have to fake using a US billing address to buy stuff from them. But you can buy books from other stores in Adobe Digital Editions ePub & PDF format, as well as DRM-free e-books. B&N also use a slightly different ePub DRM-scheme for their own books, and thus you technically have access to the broadest variety of stores to shop from.
2) The Kindle is available to Canada, but not within. All Kindles have to be ordered via the US Amazon store and shipped up here. Similarly, you buy your e-books from them at the US store, in US dollars.
Kindles use a proprietary format, and while you can get DRM-free Mobipocket books (and convert DRM-free other format e-books to work on the Kindle), for anything that's copy-restricted (like bestsellers from the major publishers), you'll either have to buy them solely via Amazon or learn to break and remove the DRM (still not illegal in Canada, despite the efforts of our current Conservative government, though I wouldn't count on that lasting much longer).
3) The Kobo is a pretty nice deal for Canadians, especially now that they've updated it to use the new Pearl e-ink screen. You can go in and try them at Chapters/Indigo stores.
The Kobo store charges in Canadian dollars and you can use Chapters gift cards to pay for your purchases, as well as use discount coupons to take the price down on much of their e-catalogue though sometimes even after coupon, the price will still be cheaper elsewhere.
You can also shop at any ADE-DRM or DRM-free ebookstore, such as the Sony Reader Store.
Kobos do have this flaw where the current firmware does not support following inline links, so if you read e-books with footnotes (academic nonfiction, for example), you're kind of screwed. But hopefully they fix that in the new firmware that goes with the Touch models. They also have this thing where the built-in dictionary can't be used with non-Kobo purchases, but again that's something they'll hopefully fix soon.
4) Sonys are kind of pricey but have nice features if you want a higher-end model. I have no personal experience with them, but as with the Kobo, you can shop at any ADE-DRM or DRM-free store.
5) Amazon has announced that they'll be supporting Overdrive library e-books (in the US, at least) sometime later this year. In the meantime, if you want to read library e-books, you'll need either an ADE-DRM supporting reader, or have the willingness and the skills to remove the DRM before you convert to a Kindle-compatible format.
Personally, as a Canadian myself, I think that either the new Kobo Touch or the Kindle would be best. And I'm leaning slightly in favour of the Kobo, which offers a good feature set at a good price, and the convenience of being available directly within Canada.
Hope this helps, and welcome to Mobile Read!