It's the quality of the printing, Cotton Bureau will be a better quality shirt too, but most importantly the art will be much more vibrant and the texture of the clothing is much more comfortable.
It's also a small start up that's local here in the states, they've got a great story and they focus purely on Tee shirts, which allows them to really focus on their craft.
Here's an example of the Pandora Seed Bank design on some packaging, not necessarily what they will be using, but it's a cool look, should they ever start selling flower