Apologies for taking a bit to get back with these, I took the photos straight away and then got sidetracked for a couple of hours. Also, they're not the greatest photos, and keep in mind that the underworks binder is at least a year old.

gc2b


Underworks

They're both very good binders, so it's hard to see a distinct difference with a shirt on (and this is a very tight shirt).
However, you can still see that the gc2b binder, because it's shorter, means there's a more distinct lump higher up. As I said, I'm sure if I had a long gc2b it would be easier to flatten out. The other thing you can notice (in the front on photo) is that it has a very distinct spot where it pulls it you can see my body awkwardly jutting back out again. The underworks is a bit longer so it tapers off after my actual chest has ended.
Another thing to mention is that after 4 years of binding, I've got mad granny blobs going on. So the fact that they're already sort of.. well flat and saggy, it helps.
All of the above being said, I strongly suggest that if you're going to choose from the two, keep in mind that they're both about as hard to put on, except I couldn't imagine you ever being able to step into the gc2b or remove it by stepping out, because the front panel does not stretch. However, the gc2b is hands down a whole lot more comfortable than the underworks, simply just because it doesn't leave you itching your armpits for the rest of the day, and I feel like the material is better in terms of getting sweaty and gross.
I've got photos of each without a shirt on, I'm just a little unsure, am I allowed to post those photos here ?