Cassettes come in all shapes and sizes. Folding bikes need a wide gear range, so stick with 11-34 or 11-36 cassettes to maximize the usable gears for hill climbing.

Customers worry about the number of gears. I would not focus on 8, 9, or 10 speed making much of a difference. The width between the biggest and smallest cog provide the gear range. 

What makes a cassette better? A cassette gives the rider gears, so there are two relevant attributes: weight, and precision of shifting. Cassettes with holes drilled out for lightness are preferable, and they look cool too. Cassettes need ramps to guide the chain up and down during shifts. Make sure the ramps look smooth and read reviews to make sure they work well. A light cassette with lots of smooth ramps is exactly what every bike needs.