The underworks website is a huge confusing mess. Here’s a summary with reference numbers that you can type into the search bar at www.underworks.com.
Econo high power compression chest binder: top 943, tank 947(long)
Cotton lined power chest binder: top 975, tank 977(long)
Cotton concealer chest binder: 988(long,half binding)
Tri-top chest binder: 983
Microfibre concealer v-tank: 996(long)
Ultimate chest binder tank: 997(long)
Microfibre conceal crew: 998(long)
Extreme Magicotton Sports and Binding Bra: 3108
Most of the binders, except the sports bra, are under men>compression shirts. The one most people talk about when they talk about underworks binders is the tri-top (983), but there are actually Options.
For the short binders, econo/943, cotton-lined/975 and tri-top/983 look pretty much the same. The econo/943 is cheaper, with only one slightly translucent layer of material (the tri-top/983 is 3 layers) and hence suitable for people with smaller chests who would not benefit much from additional binding power. The cotton/975 is also one binding layer, but lined with cotton on the inside and MUCH more comfortable than the other two, which are scratchy nylon/lycra/spandex material on both sides. The cotton/975 is lower cut than the other two, which might result in some spillover at the armpits. The tri-top/983 is A LOT stronger than the other two, and much more difficult to get on.
Comparison with gc2b: gc2b (another popular line of binders) is made of swimsuit material except the binding panel, while underworks is all-round compression. gc2b feels MUCH nicer and less rough overall with better and sturdier design, but causes back pain for me due to proportion of shoulders or something, which is why I’m trying different brands. Unlike underworks, gc2b is specifically designed for trans people, shapes better, and is more bindy than the econo/943, less bindy than tri-top/983. Price is about the same, underworks shipping (to singapore; idk about other places) costs more and is much faster.
For the full length binders, stomach compression tends to “smooth things out”, and is good for bigger chests to prevent slipping out, but can be uncomfortably constricting and tends to ride up. Consider heat! The 988 is long but only bindy in the top half and has the benefit of looking like a tank top without stomach compression. The 996, 997 and 998 are fully compressing and only differ in neckline and sleeve shape. 988 vs 997. 996/7 vs tri-top/983.
The binding sports bra/3108 is more comfortable and less bindy than the other binders, and hence may be useful for people with larger chests who don’t want that much compression, or in combination with the full length binders (3108+998, video). Sports bras don’t do anything for people with chests smaller than the built-in cups though, obviously. (As such a person, I usually wear training bras without the cups. Occasionally I use athletic tape. I don’t bind very often.)