Garden bench seat basically fall into two categories: solid and slatted.
A solid seat is the most comfortable for sitting. However, water will pool on it when it rains. It would also require drying before you sit. What's more, the water may undermine the finish of your bench over time.
As for a slatted seat, it promotes rainwater runoff. It dries quickly and it helps in preserving the finish of the product.
Bonus tip: Curved garden benches and other types of garden furniture are required to withstand lots of hardships. They're used as the focal points of gardens and patios, so they need to look great even after years of constant exposure to harsh outdoor elements. It is, therefore, best if you invest in the highest quality of chair that your budget allows. Check every area for hairline cracks, bad weds, and weak points.
As with any type of furniture, you'll be able to spend less or a lot. If you are smart with your available options, you will get the most bang for your buck.