Rectangular edges are simple to install and work well with both retaining walls and steps. While the edges of rectangles may be more susceptible to damage over time, the mold is easier to make, the concrete is easier to pour in a sharp edge shape, and the right angles of a rectangle fit in better with minimalistic and straightforward design schemes.
A stepped pattern allows you to change distance, height and spaces through a gradual reduction in layers of concrete. This design can form terraces for small flower beds. It may also create a steplike pattern or feeling to your landscaping that you can tie in with decks and patios. While this method may require more space than curving concrete, it can also allow you more design options and easier installation through stacking concrete blocks instead of spraying mortar.
If you have a basic concrete surface that you are using for your landscaping but do not like the idea of it curving, considering using face brick or veneers on the surface. This only works for walls and steps, but it can allow you to mortar natural stone or brickwork onto your project. These materials provide warmer colors that have a better chance at blending with your yard. You will need to install them with meshwork and moisture drainage spaces to avoid future damage.
Consider skipping concrete altogether. If you do not want to go to the time and expense of curving your concrete, buy a metal mold instead. There are copper and steel walls designed for landscaping curved borders on flowerbeds, curved retaining walls and other projects.