Honestly, if you have ever sat on a park bench right after a summer rain, you already know the answer. A bench with gaps between the slats will almost always drain water faster and dry out sooner than a solid one. But let me walk you through the reason, because it is not just about the obvious gaps.
A solid park bench, whether it is made of metal, recycled plastic, or a single wide board, has a completely flat and continuous surface. When it rains, water has nowhere to go except to pool on top or run slowly to the edges. Even a slight slope or a worn-out spot can leave puddles that stay for hours. That means you get a wet seat and, worse, a wet spot that soaks into your pants the moment you sit down. Over time, standing water also encourages mold, mildew, and in the case of wood, faster rot and cracking.
A slatted bench is different. Each gap between the slats acts like a tiny drain channel. Water hits the top, runs down the slight curve or bevel of a slat, and drips through the open spaces to the ground below. Because air can circulate through those gaps from both top and bottom, evaporation happens much faster. In my experience, a good slatted bench can go from soaking wet to reasonably dry in under an hour on a breezy day, while a solid bench can stay damp well into the afternoon.
That said, not all slatted benches are created equal. The width of the gaps matters. If the gaps are too narrow, say less than a quarter of an inch, surface tension can hold water in the crack and actually slow drying. If the gaps are too wide, they become uncomfortable to sit on and can catch heels, keys, or even small fingers. Most well-designed park benches use a gap of about half an inch to three quarters of an inch. That is wide enough for water to pass through easily, but narrow enough that you do not feel like you are sitting on a ladder.
Another thing people forget is the material. Wood slats, especially cedar or teak, absorb some water but dry quickly because of the air gaps. Recycled plastic slats do not absorb water at all, and with gaps they dry almost instantly. A solid recycled plastic bench can still hold puddles because the water has no exit path except running off the edge.
So yes, as a general rule, park benches with gaps between the slats drain water better than solid ones. If you are choosing a bench for a rainy climate, near a splash pad, or by a pool, go for slatted. If you want something easier to clean with no cracks for crumbs and leaves to get stuck, a solid bench might be fine, but you will need a towel after rain. I would pick the slatted one almost every time.