Wal-Mart. It's only fairly recently that Target stores began being built in Canada, so there aren't all that many of them as of yet. As a result, I have not had very many opportunities to go into a Target.
I think I prefer Wal-Mart because it's more familiar to me. Each store I've been to was laid out the same, so it was easy to find what I was looking for. The private brand that Wal-Mart sells (Great Value) is cheaper than most other brands, and isn't all that bad. Also, modern Wal-Marts (known as "Supercentres") are department stores with a full grocery section included. There isn't a Wal-Mart in my town, but there is in one nearby, and we buy our groceries there whenever we can because it is much cheaper than the grocery stores around here.
Maybe if Target were more familiar to me, my opinion would be different; however, since I have not seen it much, my opinion is definitely going to go toward the one I visit often.