Many flowering aquatic plants will require full sun to do their best. Conversely, the bright sun that helps the plants thrive will contribute to algae bloom, and heating of the water. Overheating of the water can be a concern for fish. Plan to budget for lilies and extra plants if you do locate in the sun. Plan on extra maintenance if you locate near trees.
Formal designs utilize geometric and symmetric shapes.
Informal shapes are free-form are assymetric. The more complex the shape, the harder it is to construct and maintain. In general, stick with a shape that will flow best with the rest of your garden. In a smaller yard, the space itself may dictate the shape, but formal shapes usually have a larger appearance. The pond will tend to look larger on paper, or out of the ground, than in.