[ATDrake]

Your parents will also need to have a US billing/shipping address (not necessarily their own) to put in their account on B&N's website before they can buy books. Downloading them from within Canada isn't a problem, but B&N effectively only allow checkout to US-based customers, or at least IP-addresses which look like they are.

Another thing that may be a consideration is library e-books. Many places in Canada (especially the big metropolitan areas) seem to have a pretty good selection. They mostly come in ADE-DRM ePub and PDFs, which Sony, Nook, Kobo, etc. support out of the box, but the Kindle doesn't.

As for your other concerns, you can share books between multiple Kindle, Kobo, Sony, or Nook devices registered to the same account. For Sony and Kobo, it all depends on the Adobe Digital Editions registration, which allows up to 6 devices per user ID. Once your parents' readers are registered, they can easily put their books on either or both devices and read them simultaneously.

Kindles have an additional bonus in that people can keep separate accounts, but swap their collections by temporarily registering their Kindles to the other person's account just long enough to download the book they want.

Nooks take it a step further in allowing anyone to enter the unlock code for their books just once on another person's device, which thereafter stores it so they can read any other books from the same source, as well as the books that go with the original user's account. Just a warning: the official Nook lending method is actually a one-time-only, 14-days-only feature which depends on publisher restrictions.

Regarding book availability and shopping ease, there's nothing special in particular about books not being available for Canadians in the Kindle store. Any titles which for some reason or other aren't available to Canada due to rights issues or whatever will be unavailable in pretty much all the stores.

Some people get around this by creating a separate US account for purchases they want to make, but this may be too much of a hassle for your parents so maybe the Nook with a B&N store account would bypass this, if you think it's something that might turn up enough to be bothersome. They can still shop at the Kobo store for any Canadian-only titles they might be interested in, as long as it's a title available in ePub, since the Nook supports the ADE-DRM that the Kobo store uses.

However, apparently the Nook insists on sideloading any non-B&N purchases into the My Documents section instead of My Books, which your parents may or may not find confusing (some people do, not unreasonably expecting *all* their books to be listed under "My Books").

For actual buying, I think I'd say that Kobo would probably end up having the best pricing overall. While it doesn't get some of the deep discount deals that Amazon occasionally features, they do regularly put out coupons for $1-2 or 20-30% off, which work on most books for Canadians. And I've read that the Kobo people are working on getting Chapters gift certificates to work on the site.

Also, the Kobo site charges in Canadian dollars, whereas the Amazon Kindle store, even when set to Canada, will charge in US dollars. Though you can minimize the currency conversion fees by buying a large denomination gift certificate to apply to your parents' account when the exchange rate is good.

Finally, the Kindle is very easy to use, and 3G works fairly well in Canada (at least, in the suburban satellite municipality of the major metro area where I live) and should work in the States when they visit.

Hope this helps, and welcome to MobileRead!