Yes, I understand they are different models, but informally both are referred to as Cubes (obviously due to the shape of the case.) In fact, if you look on ebay, the first "NeXT Cube" I found was actually a "NeXT Computer." I believe they may both actually have the same N1000 model number, despite CPU differences.

The faster 68040 motherboards were available as an upgrade for the older 030 models. So it's not like a "NeXT Computer" couldn't have the 040 inside.