I’ve sat on both enough times to say the comfort gap is real, but it depends on how long you’re sitting and how your body is built.
A flat slat park bench is basically a straight wooden or metal shelf. It doesn’t follow the shape of your body. For the first few minutes it’s fine. After that, the hard edges of the slats start pressing into your thighs and tailbone. Because the backrest is usually straight or only slightly tilted, your lower back gets very little support. You end up slouching or holding yourself upright with your core. If you have shorter legs, the front edge may dig into the back of your knees and cause tingling. The advantage is freedom: you can sit sideways, cross your legs, or stretch out without fighting the shape.
A contoured bench has a curved seat pan and a backrest shaped to follow the spine. The seat cups your hips slightly, and the backrest gives your lumbar area something to lean into. That spreads your weight more evenly and reduces hot spots. For most body types, a contoured bench feels noticeably better after 15 to 20 minutes, especially for the lower back. The downside is that the curve is one-size-fits-most, not one-size-fits-all. If the contour hits the wrong part of your back, it can feel worse than a flat bench. It is also less forgiving if you like to sit sideways, cross-legged, or lie down.
So the short answer: flat slat benches work best for short, casual, move-around seating. Contoured benches win for longer sits, lower-back support, and overall pressure relief. If comfort is the main priority, a contoured bench is usually worth it.