True mexican weeping is otatea acuminata aztecorum.

Acuminata acuminata grows to about 2/3 the size, and looks a lot like mexican weeping - purple culms at new growth, graceful with long slender leaves. Leaves aren't quite as long, but fairly proportionate in relation to size.

It's just my experience, but ac.ac. doesn't spread as much as ac.az. Ac.az. has a habit of spreading a bit quicker than most 'clumping' bamboos.