Hello! NB binder wearer here. As Al-the-grammar-geek mentioned, Underworks and GC2B are the most well known and popular binder manufacturers.
Underworks binders and designed primarily for cis men with gynomastia
GC2B binders are made specifically for trans men
Which you find more comfortable will depend on your own body type. For me personally, on the whole Underworks binders tend to be slightly more comfortable/less restrictive with a little more leeway on sizing. Their armpit holes are a little higher up (my armpits are a binding problem area) and the back panel is more sheer so more stretchy.
In general, I hear most people find GC2B binders more comfortable. My first one (abt 3 years ago) didn’t work out so well for me, when new they are very restrictive indeed and difficult to put on, the back panel has far less stretch. My weight has changed a little since the last time I owned one, and I believe the design has also slightly changed, so when I bought a new GC2B binder after Al gave me a gift card I was a little sceptical after my last experience but although I still have a lot of armpit overhang the binding part itself is comfortable once on, though still very hard to get on and off. But this is just my personal experience, on the whole, I hear most positive recommendations for GC2B and my body type is a bit of an outlier, I think. The GC2B binder is still very comfortable though, and I know their returns policy is very good regarding needing to change sizes, but definitely put thought into which size to go for when ordering from GC2B.
Not very much separates them, so as I say which works best for you will be personal based on your body. In general, my main recommendations would be:
1) For your first binder, go for a half tank (gc2b)/extreme tri top (underworks) - whichever brand you go for. Binders take some used to getting on and off, because you can’t easily get them on and off like regular clothing and they’re prone to getting stuck especially at first, when new, unwashed, and at their most tight. Full tanks are a lot more restrictive (because they go down to your waist) which can be quite scary as your first experience if you get stuck (speaking from my own first experience!!). Don’t be surprised if you do get stuck at first, even with a half tank! It happens, but they do get looser with wear and washing. Just take a deep breath, feel for the bottom seam, and keep pulling. Half tanks are also a lot cooler in summer, because all binders are made of not very breathable stretch material.
2) Measure yourself well, and accurately, with a fabric tape measure. If you’re close to the upper boundary on a size, go the next size up. For your body, binding looser is better than binding too tightly (no matter what dysphoria might say), because binding too tightly can lead to rib damage, bruising, and permanent disfigurement of your chest muscles (which, I don’t know if you’re wanting top surgery at some stage, but chest tissue damage can prevent you from being able to have top surgery)
3) Listen to all the safety instructions you’ve ever read online about all binders because they’re not an exaggeration! 6-8hrs maximum wear per day (the less the better, especially if you're new to binding, ease yourself in with an hour or so at a time), take breaks if you can, cough regularly, take a deep breath when first putting the binder on, lift your arms above your head regularly to stretch your back and expand your ribcage, sit/stand up straight as much as possible, and don’t be afraid of talcum/baby powder under the chest/around your armpits, because when its hot, chafeing and rashes are super common because of the type of material. Unsafe binding can cause injuries which can't be corrected so always listen to your body.
Hope this all helps. Finding one that fits takes a little trial and error but once you have one that fits it is a life changing experience.
