Bent shaft paddles allow for easier paddling over long distances and on smooth flat water can help you attain very good speed. Four miles an hour in a 17 foot canoe is easily achieved in flat water if you and your partner are paddling efficiently and are in average health. Straight shaft paddles are more of the traditional canoe paddle. You don't have to spin the paddle around 180 degrees in your hand if the blade angle is facing the wrong way. If you plan to do any moving water above Class II-, you should use straight shaft paddles.
Paddle length is also very important.
You can not gauge the proper length of your paddle by walking into the store and finding what is comfortable. You will be paddling your canoe sitting, not standing. The best way to gauge how long of a paddle you should get is to get into your canoe, and measure the distance from the nose to the waterline. This is how long your shaft should be. Paddles come in a wide variety of materials, sizes, styles, and prices.