I've got to agree with Lynx-lynx. Books on Board are the best. I pay USD prices in pounds sterling from their UK site and not UK prices which are generally more expensive even with discounts. They also have a Canadian site with prices in Canadian dollars. Give it a go.
I don't like Kobo as you can only buy books one at a time. There's no cart so you can't buy multiple titles in one go and they also don't have a wish list.
As for Sony they're expensive and their selection, in the UK at least, is not very good compared to Books on Board and Kobo.
My advice is to shop around. Don't limit yourself to a single source. I browse through 6 retailers when I'm buying books. You never know when someone might have offers or a sale on.