You can get good flow with either design. With a rectangular pond, you can use TPR's (tangential point returns) to eliminate the dead spots. And it is possible to get dead spots on the perimeter of a round pond.
Coriolis force is too weak to have significant influence on the direction water spirals to a drain. It's the geometry of the drain and pond, sink, or tub that determines which way the water will go.
As to which shape I'd build, it really depends on the site. In some sites, you might actually have room for more water with a round pond. Also, other shapes are possible. An oval, for instance, or two semi-circles connected by straight walls. If you're going to use an EPDM liner and want to keep folds to a minimum, though, you'll want to stick with a rectangle. OTOH, if you go deep, straight walls require more reinforcement.