Honestly, if you're furnishing a rooftop terrace, the first thing I'd skip is anything made from solid steel, cast iron, or thick hardwood unless you never plan to move it. The sweet spot for a rooftop is aluminum. Powder-coated aluminum benches are light enough to carry upstairs or shift around when the sun moves, and they won't rust if rain blows in.
The style I'd look for is an aluminum slat bench. The slatted seat lets water drain instead of pooling, and the open design reduces wind load, which matters more on an exposed rooftop than people expect. I'd go for a low, modern profile in matte black, white, or a wood-look finish. A bench around 20 to 30 pounds is ideal: heavy enough to stay put in a breeze, but still easy for one person to lift.
Another good option is resin wicker over an aluminum frame. It has a warmer, lounge-like feel than metal slats, but check that the frame is actually aluminum rather than steel, otherwise you're adding unnecessary weight. The main trade-off is cushions, since they're comfortable but need to be stored in bad weather.
If you want a wood look, I'd only choose eucalyptus or acacia if you're okay with sealing it and carrying a bit more weight. Solid teak is beautiful, but it can be heavier than you'd want for a rooftop and costs more than you need to spend. Poly lumber benches made from recycled plastic are also weatherproof, but many are surprisingly heavy, so check the listed weight before buying.
My honest recommendation is to start with a powder-coated aluminum bench with a slatted seat. It's low-maintenance, easy to move, modern enough for city views, and less likely to damage rooftop surfaces than heavier furniture. Add a small aluminum side table and you'll have a setup that's easy to rearrange when you're cleaning or entertaining.
If you tell me your budget and whether you want a backrest or a backless bench, I can narrow it down further.