Ecologically speaking, bamboo grows way faster than trees, so the obvious choice for renewable lumber is bamboo. But on the other hand, trees are better for the soil (generally speaking) and do amazingly good things for the water cycle, plus put off more oxygen than grasses (again, generally speaking). So to grow trees for lumber is actually a good thing for the environment. In this case I'd say it's six of one and half-dozen of the other, in terms of which is better.
But that's not what I was thinking when I said there were botanical reasons bamboo is better than wood for knitting needles. Take a gander at our lovely illustration.
It's too small to read the details, but really, all I wanted to show was how plant and animal cells are different. See how the plant cell has walls? It's square-ish? Plants build walls between each cell, which gives everything structure, like a wall of bricks. Animal cells, on the other hand, have no walls and are watery blobs that need skeletons or else they ooze around like octopuses. (Octopii? Squids. Like squids.) Cell photo from here.
All rightie. So plants have a skeleton, so to speak, at the cellular level. For some plants, like grasses, that's the only structure they have. Other plants, like trees, produce what science types call "secondary growth" or "woody growth" which is, yes, wood and acts as a skeleton for honkin' big trees. (To some degree, that is where tree rings come from.) Grasses rely entirely on their little cell walls for structure, wood has cell walls and woody growth.
This means, to a knitter, that the material bamboo knitting needles are made out of is much finer, smoother, and more flexible than wooden needles. I guess it's really a matter of opinion which one of them is better, depending on what you're looking for in your needles. I think bamboo is better due to the slight flex, but then that's just my opinion.
But at any rate, that's what makes wood and bamboo needles different. I have left out a bunch of stuff involving primary, secondary, and apical meristems, and have totally left out the intercalary meristems which have no relevance but which I think are cool. Hope it wasn't too painful.
Now if you'll excuse me, I have a small war to fight with some purple food coloring.