Rounded is better for aero. Sloped top means surface area on each side decreases front to rear while pressure on top also decreases. Side pressure needs to relieve itself with flow over the top. Unless, the sides are tapered as well. No rollover flow requires balanced pressure. In theory that is possible but tricky. In practice cross winds introduce asymmetry. So a generous radius is your friend aero wise.
Cargo wise, sharper corners only help if what you carry needs those precious few inches due to combined length, width and height. Bondo provides a clever solution. With his lid in the up position soft sides make it behave like a conventional topper with full rear height and width. Best of both worlds.
Ref:
this post.