You can feel the market by how easy it is to move up ranks. In the Amazon market, the US seems the highest saturated, UK comes second, then Australia and, probably, last is Canada. This means that competition is more fierce in the US Amazon market, but it also means that more readers are available to buy your book there.
In Kobo, it's flipped. Canada is their largest market and the US is the most scant.
I don't have a great feel for Apple's market. I'm betting it's similar to Amazon. B&N is US. GooglPlay is GooglePlay. (I've given up advertising with Googleplay. For some reason, I can get lots of clicks for googleplay books with ads without any sales).
I wouldn't turn away from the Amazon Canadian market. The disadvantage is there's less readers, but it's also easier to keep a book hovering on bestseller lists. And sometimes ads work better in smaller markets because of less competition.