As long as our influence is not militaristic, I think it is in our best interest to encourage capitalism and free markets in the Americas. The more we can build up our neighbors to the south of us, to be like our neighbor to the north of us (economically speaking), the better it is for all of us.
From what I understand about China and Africa is, they just came in an either offered deals that had to be accepted or they just took what they want. Until we start mining the asteroid field, we are in a race for control over rare earth elements and China is winning.
As far as Venezuela goes. It is collapsing, which means that someone needs to step in and support the country during the transition. If we don't do it, China or Russia will.