It doesn’t sound trivial to me, that’s why I’m bringing it up. 
I’m a UX designer as well.
I’m just saying that it might not be »sometimes space« vs »sometimes backspace« but rather »very often space« vs »sometimes backspace« in addition to »bunch of related platforms with @mentioning also insert space so people are used to it«.
But it’s your call, and a good point with the space bar being way closer. 