In short, rounded hulls are more predictable and controlled than flat ones on all types of water. The round and Vee bottom hull excels in every category except initial stability. A canoe should have high final stability plus enough initial stability so you can paddle it without fear of capsizing. Later, when your friends have dried out you'll swear that your superiority in rapids is due to your impeccable paddling skill rather than a smooth bottom canoe. Let's not mince words.
External keels are an inferior canoe design.
Aluminum canoes are formed in two halves, so they need a keel to hold the halves together. But even here, the keel could be mounted on the inside of the hull rather than the outside. The real reason for keels is to stiffen a floppy bottom. Avoid canoes with keels. Shoe keels make a lot more sense than the standard T-grip rock grabbers.